or. EORAVENTURE 
CHEN. ISTRY LIBRARY 


H 


THE CHEMISTRY OF _ 
LEATHER MANUFACTURE 


BY 


JOHN ARTHUR WILSON 


CHIEF CHEMIST, A. F. GALLUN & SONS CO., MILWAUKEE, WIS.; CHAIRMAN, 
LEATHER DIVISION, AMERICAN CHEMICAL SOCIETY 


American Chemical Society 


Monograph Series 


mek DE PAR TV NT 


The CHEMICAL CATALOG COMPANY, Jne. 
19 EAST 24TH STREET, NEW YORK, U. S. A. 
1923 


CoPYRIGHT, 1923, BY 
The CHEMICAL CATALOG COMP “ 


All Rights Reserved 


Press ret 
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GENERAL INTRODUCTION 


American Chemical Society Series of 


Scientific and Technologic Monographs 


By arrangement with the Interallied Conference of Pure and Applied 
Chemistry, which met in London and Brussels in July, 1919, the 
American Chemical Society was to undertake the production and pub- 
lication of Scientific and Technologic Monographs on chemical subjects. 
At the same time it was agreed that the National Research Council, 
in codperation with the American Chemical Society and the American 
Physical Society, should undertake the production and publication of 
Critical Tables of Chemical and Physical Constants. The American 
Chemical Society and the National Research Council mutually agreed 
to care for these two fields of chemical development. The American 
Chemical Society named as Trustees, to make the necessary arrange- 
ments for the publication of the monographs, Charles L. Parsons, 
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The development of knowledge in all branches of science, and espe- 
cially in chemistry, has been so rapid during the last fifty years and 

3 | 


4 GENERAL INTRODUCTION 


the fields covered by this development have been so varied that it is 
difficult for any individual to keep in touch with the progress in 
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It was with a clear recognition of the usefulness of reviews of this 
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auspices of the Society. 

Two rather distinct purposes are to be served by these monographs. 
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The publication of these books marks a distinct departure in the 
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regard to commercial considerations. The success of the venture will 
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BOARD OF EDITORS 


Technologic Series :— 

IAM. DA. Noves, Editor, Harrison E. Howe, Editor, 
: (ERICK: 

WILLIAM HOSKINS, 

F, A. Lipsury, 

ARTHUR D. LITTLE, 

Cia REESE, 

C. P. TOWNSEND. 


American Chemical Society 


MONOGRAPH SERIES 


Other monographs in the series of which this book is a part now 
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Organic Compounds of Mercury. 

By Frank C. Whitmore. 397 pages. Price $4.50. 
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Valence, and the Structure of Atoms and Molecules. By Gil- 
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Aluminothermic Reduction of Metals. By B. D. Saklatwalla. 

Absorptive Carbon. By N. K. Chaney. 

Refining Petroleum. By George A. Burrell, eé al. 

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The Animal as a Converter. By H. P. Armsby and C. Robert 
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The Properties of Metallic Substances. By Charles A. Kraus. 

The Structure of Crystals. By Ralph W. G. Wyckoff. 

Physical and Chemical Properties of Glass. By George W. 

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Photosynthesis. By H. A. Spoehr. 

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PREFACE 


The chemistry of leather manufacture is progressing more rapidly 
now than at any previous time. Much of the earlier work fasled to 
recognize the existence of important variable factors and has been 
rendered obsolete by recent investigations carried out under more 
highly refined conditions. In preparing this monograph, it was found 
necessary, for the purpose of correlating existing data, to conduct many 
special investigations and these are being reported here for the first 
time. Advance information on investigations under way in other 
laboratories has been obtained, wherever possible, so that the presenta- 
tion might be made reasonably complete to the close of the year 1922. 

The literature pertaining to leather manufacture is so vast and the 
views expressed so numerous and divergent as to make an impersonal 
compilation of all published papers encyclopedic in size, bewildering to 
the average reader, and an undertaking of questionable value. In order 
to fulfill the first purpose of this series of monographs, namely, to 
present the knowledge available in a readable form, intelligible to those 
whose activities may be along a wholly different line, the author has 
felt compelled to present the subject from his own viewpoint, making 
no attempt to discuss views which, in his opinion, fail to contribute 
anything to the development of leather chemistry. In so doing, the 
author is fully aware that there are others who do not share his opinions 
of the relative merits of various views, but he can only admit his 
inability to present adequately views which appear to him unsound. 
But, ina field so vast, there is ample room for as many volumes as there 
may be sides to the question worthy of presentation and it is in the 
preparation of additional volumes that criticism of this attitude may 
find its best expression. 

A considerable amount of space has been devoted to the histology 
of skin and to the physical chemistry of the proteins because of their 
fundamental bearing on the chemistry of leather manufacture. Descrip- 
tions of analytic methods and practical details of leather manufacture 
have been given only where they seemed necessary to make the subject 
clearer to chemists unfamiliar with tannery routine. 

Many of the ideas presented in this book were gained during a 

7 


8 PREFACE 


period of intimate association with Professor H. R. Procter, of the 
University of Leeds, England, who is affectionately known throughout 
the world as the “father of leather chemistry” and whose books on 
leather manufacture have ‘been the standard for the past thirty-five 
years. 

In the preparation of sections and photomicrographs, valuable assist- 
ance was rendered by Mr. Guido Daub, whose painstaking efforts are 
largely responsible for the success of this phase of the work. The 
sections and specimens of human skin were procured from Professor 
T. H. Bast, of the University of Wisconsin. Professor Arthur W 
Thomas, of Columbia University, supplied the skins of guinea pigs and 
albino rats fixed in Erlicki’s fluid. Leathers from the hides of the 
hippopotamus, walrus, and camel were furnished by Professor Douglas 
McCandlish, of the University of Leeds. Most of the remaining speci- 
mens were provided by the firm of A. F. Gallun & Sons Company, in 
whose laboratories the work was done. The interesting photographs 
illustrating the drying of gelatin blocks were furnished by Dr. beige 
Sheppard, of the Eastman Kodak Company. 

Grateful acknowledgment is made of the generous criticisms and 
suggestions given by Mrs. Marion Hines Loeb, of the University of 
Chicago, on the general histology of skin; by Dr. Jacques Loeb, of the 
Rockefeller Institute, on the physical chemistry of the proteins; and by 
Professor A. W. Thomas, Mr. Frank L. Seymour-Jones, and Miss 
Margaret W. Kelly, of Columbia University, on many important points 
throughout the book. 

The author is most deeply indebted to the late Arthur H. Gallun, 
whose devotion to the cause of leather chemistry has made arate a 
large portion of the data presented in this book. 


jae 
Milwaukee, Wisconsin, 
March 12, 1923. 


CONTENTS 


CHAPTER I.—INTRODUCTION it Sa Roe ee, eee 


CHAPTER 2.—HISTOLOGY OF SKIN 


Preparation of Sections and Photomicrographs for Study—General 
Histology of Skin—Cow Hide—Calf Skin—Sheep Skin—Goat Skin— 
Hog Skin—Horse Hide—Guinea Pig Skin—Fish Skins—Other Skins. 


CHAPTER 3.—CHEMICAL CONSTITUENTS OF SKIN . 


CHAPTER 4.—IONIZATION OF ACIDS AND BASES CoMMONLY USED IN THE 
EMERY.) oc. . 


Acids—Bases—Order of Strengths—Temperature—pH Values—Effect 
of Added Salts. 


CHAPTER 5.—PHYSICAL CHEMISTRY OF THE PROTEINS . + + + ¢ 


Donnan’s Theory of Membrane Equilibria—Swelling of Protein Jel- 
lies—The Acid-Protein Equilibrium—Repression of Swelling by Salts 
—The Alkali-Protein Equilibrium—Two Forms of Collagen and Gela- 
tin—Electrical Potential Difference between Protein Jelly and Aqueous 
Solution—Rhythmic Swelling of Protein Jellies—Structure of Gelatin 
Solutions and Jellies—Relation of the Osmotic Pressure and Viscosity 
of Gelatin Solutions to the Swelling of Gelatin Jellies—Osmotic Pres- 
sure and Membrane Potentials—Changes in Viscosity of Gelatin So- 
lutions with Time—Theory of Salting Out and the Stability of 
Colloidal Dispersions—Adsorption. 


CHAPTER 6.—PRESERVATION AND DISINFECTION OF She RCP ee ee bee ek ae 
Salting — Salt Stains — Drying — Salting and Drying — Pickling — 
Disinfection. 

OpArteR 7. SOAKING AND FLESHING . . - + + + + © © § 

CHAPTER 8.—UNHAIRING AND SCUDDING ...- .- 


Sweating—Liming—Plumping and Falling—Fresh vs. Mellow Lime 
Liquors—Unhairing by Means of Other Alkalies—Unhairing by Means 
of Acids—Unhairing by Means of Pancreatin—Combined Bating and 
Unhairing by Means of Pancreatin. 


CHAPTER 9.—BATING 


Falling—Regulation of Hydrogen-lon Concentration—Deliming—Bac- 
terial Action—Enzyme Action and Elastin Removal—FEffect of Hydro- 
gen-Ion Concentration—Effect of Time of Digestion—Effect of Con- 
centration of Enzyme—FEffect of Concentration of Ammonium Chloride 
—Distribution of Elastin Fibers in the Skins of Different Animals— 
Effect of Elastin Removal on the Final Leather—Digestion of Col- 
lagen during Bating. 


9 


PAGE 
Il 


15 


65 


76 


04 


133 


142 


151 


173 


10 CONTENTS 


CHAPTER 10——-DRENCHING AND PICKLING. . . ©. -«.” sunigula eee 


CHAPTER II.—VEGETABLE TANNING MATERIALS ... ba 


Classification—Sources of Tanning Materials—Leaching—Effect of 
Temperature—Effect of Hardness and Alkalinity of the Water—Effect 
of pH Value on the Color of Tan Liquors—Effect of pH Value on the 
Oxidation of Tar Liquors—Effect of pH Value on the Precipitation 
of Tan Liquors—Clarifying, Decolorizing and Drying. 


CHAPTER I2.—THE TANNINS nr 
Practical Definition of Tannin—The Gelatin-Salt Test for Tannin— 
The Determination of Tannin—A. L. C. A. Method—Wilson-Kern 
Method—Comparison of A. L. C. A. and Wilson-Kern Methods—Effect 
of Washing—Conversion of Nontannin into Tannin—Effect of Aging 
—Effect of pH Value—Modified Wilson-Kern Method—Potential Dif- 
ference of Tannin Solutions—Isoelectric Points of the Tannins—Pre- 
cipitation of Tan Liquors. 

CHAPTER 13—VEGETABLE TANNING . .. . 


? 


The Structures of Tanned Skins—Rate of Diffusion of Tan Liquor 
into Gelatin Jelly—Rate of Tanning as a Function of Time and Con- 
centration of Tan Liquor—Rate of Tanning as a Function of pH 
Value—Stability of the Collagen-Tannin Compound at Different pH 
Values—Effect of Neutral Salts upon the Rate of Tanning—Degree of 
Plumping of Skin as a Function of Concentration of Acid and Salt 
in Tan Liquors—Rapid Tannages—Theory of Tanning—Procter-Wil- 
son Theory—Oxidation Theory. 


CHAPTER I14.—CHROME TANNING . 


Chromium Collagenate—Hydrolysis of Chromium Salts—Diffusion of 
Chromium Salts into Protein Jellies—The Time Factor in Chrome 
Tanning—The Concentration Factor in Chrome Tanning—Effect of 
Neutral Salts upon Chrome Tanning—Effect of Salts of Hydroxy- 
Acids upon Chrome Tanning—Comparison of Chrome and Vegetable 
Tanned Leathers—Theory of Chrome Tanning. 


CHAPTER I5.—OTHER METHODS OF TANNING 


Combination of Chrome and Vegetable Tanning—Alum Tanning—Iron 
Tanning—Tanning with Colloidal Silicic Acid—Miscellaneous Mineral 
Tannages—Tanning with Oils—Tanning with Aldehydes and Quinones 
—Tanning with Syntans. 


CHAPTER 16,—FINISHING AND MISCELLANEOUS OPERATIONS . .. . 


Bleaching—Stuffing and Fatliquoring—Penetration of Dispersions 
through Grain Surface—Fatty Acid Spews—Coloring—Finishing. 


213 


240 


278 


309 


322 


THE CHEMISTRY OF LEATHER 
MANUFACTURE 


Ghapter “I. 


Introduction. 


Leather chemistry is one of the most fascinating branches of indus- 
trial chemistry and also one of the most complex, dealing, as it does, 
with reactions between those poorly defined groups of substances, 
usually colloidal, whose compositions are still matters for speculation. 
The raw skin is composed largely of various kinds of protein matter 
and is complicated by a structure which varies considerably in differ- 
ent animals and even in different parts of the same skin. Conversion 
into leather involves the removal of some of these proteins by the 
action of alkalies, enzymes, or bacteria, and the interaction of the re- 
mainder with tanning materials, oils, soaps, emulsions, mordants, dye- 
stuffs, gums, resins, and other complex materials. During these 
reactions the structure of the skin must be carefully preserved, or 
improved, and highly developed technic is required to impart to the 
resulting leather certain necessary, but almost indefinable, properties, 
many of which it is an art even to appreciate fully. When one con- 
siders the vast amount of energy expended by organic chemists upon 
the materials involved in making leather and the uncertainty of our 
knowledge concerning the individual substances, the complexity of the 
whole problem becomes more apparent. 

Leather manufacture as an art probably antedates chemistry as a 
science. Well preserved specimens of leather from ancient Egypt bear 
testimony to the high state of development of the art over three thou- 
sand years ago. Its origin presumably dates back to the time when 
man first began to kill animals for food. The skins, not being palatable, 
were very likely discarded at first, but the value of dried skins for 
clothing, or protective covering, could hardly remain long undiscovered. 
Dried skins are hard and stiff, but would become considerably softer 
and more pliable after being bent and worked during use, and it 
was probably noticed very early that this softening action is more 
pronounced if the skins are worked while being dried, especially in 
the presence of fats, such as would naturally cling to the skins of 
animals crudely flayed. In rainy seasons, when the skins could not 

II 


12 THE CHEMISTRY OF LEATHER MANUFACTURE 


be dried rapidly, putrefaction of the epidermal cells would cause the 
hair to slip and reveal the advantages of unhaired skins for certain 
purposes. The tanning and coloring actions of leaves, barks, and 
woods were probably also accidental discoveries of a prehistoric age. 
In fact, many of the tannery operations in use today are of ancient 
origin. | 
Secrecy and lack of accurate records make it difficult to follow the 
evolution of the art, especially in the matter of details essential to the 
production of the finer qualities of leather. But developments have not 
all been made by rule-of-thumb methods, as has often been supposed. 
The great success of a certain class of tanners, for example, has been 
due to the development of a science of leather manufacture, as distinct 
from the art, based upon a belief in the constancy of natural laws 
and involving the organization and classification of countless facts 
gained by experience or handed down from previous generations. This 
science, because of its high degree of specialization, has proved more 
powerful in a practical way than chemistry, so much so that chemistry 
must still be regarded as of value primarily in supplementing and not 
replacing the science of the tanner. 

Disillusionment has been common among chemists entering this 
industry, as the result of the unexpected intricacy of the application 
of chemistry to leather manufacture, of insufficient training, of false 
notions of superiority over artisans who had devoted their lives to 
the industry, or of failure to appreciate that the tanner’s own science 
is usually far more reliable than the chemistry of a beginner in the 
industry. 

~In order to make substantial progress, the chemist must, asa rule, 
devote himself completely to a study and explanation of the mechanism 
of each step of a process already in successful operation and without 
in any way interfering with the operation of that process. Once avail- 
able, sound explanations of the mechanism of existing processes are 
of incalculable value in suggesting practical experiments leading to the 
elimination of unnecessary operations and to the improvement and 
development of others. 

That this procedure has not been more widely adopted is easily 
explained. Long and costly studies are required for which there is no 
immediate return, and whether there will ever be a return commensurate 
with the cost of the studies must depend upon the skill of the chemist, 
which it is difficult for the tanner to judge. Moreover, the qualifi- 
cations required of the chemist are extremely severe. He must have a 
broad, theoretical training, marked ability to advance the pure science, 
great skill in adapting delicate apparatus to crude, tannery conditions, 
and power to appreciate the viewpoint of a successful tanner. Previous 
contact with the industry, on the other hand, is not essential. 

That close codperation between the university and the industry 
would be highly profitable to both cannot be denied, but there is little 
chance of such cooperation being brought about until each acquires. a 
better understanding of the needs and potentialities of the other. The 
stumbling block has been either the failure to appreciate the value 


INTRODUCTION 13 


of cooperation or the disinclination of one or the other to take the 
initiative. 

The university can derive at least three important lines of ad- 
vantage from cooperation with the industry, the most obvious of 
which is much needed financial support. But the laws af-the; con- 
servation of mass and energy hold in industry, as in everything else. 
The university cannot continue to receive from the industry with- 
out returning a like amount, although this may be of a different kind. 
The university has vast resources of potential wealth, but it suffers 
from having too little in liquid form. But industry constitutes a 
means of converting one form of wealth into another. The university 
can be assured a continuous financial support from the industry, but 
only by supplying the industry with the means of producing this wealth. 

Another advantage to be gained by the university is the view- 
point of the industry, which is necessary for the university to prepare 
its potential wealth so that it may be assimilated by the industry. This 
viewpoint will also help the university to train its students to make 
a greater success in industry. The third advantage lies in the fact 
that the industry offers a field of employment for the students of 
the university. The industry is always in need of men _ properly 
trained from its own viewpoint. But, too often, the training which 
men receive at the university does not equip them with a power 
of service to the industry that is in demand at all times. Through 
closer cooperation a system of training could be devised that would 
guarantee the opportunities of industry to men with initiative and 
ambition. | 

The source of wealth that codperation offers to the industry con- 
sists of fundamental data and of men trained to apply these data 
to practical production. The possibilities for increasing efficiency in 
the industry are almost unlimited and so are the profits to be 
derived by both the university and the industry from intelligent 
cooperation. 

Chemists within the industry have always been handicapped by lack 
of fundamental information. Many of the physical properties that 
determine the value of leather are determined by its microscopic 
structure, but very few tanneries have found themselves in a position 
to develop the means for studying the histology and chemical constitu- 
ents of skin and the structure of leathers made under different condi- 
tions. Such studies are expensive and time consuming and their devel- 
opment in each individual tannery would be very extravagant. The 
_ industry could well afford to finance a laboratory to be devoted solely 
to such studies, which would probably cover a period of many years. 
That a good start has been made will be evident from a perusal of the 
next chapter. 

The physical chemistry of the proteins is a subject of fundamental 
importance to leather chemistry, but, since it is also of fundamental 
importance to many other branches of chemistry, it should be pos- 
sible for some good university laboratory to establish a great scheme 
for co-operative work in this field, drawing financial support from 


14 TITHE CHEMISTRY OF LEATHER MANUPACIU Re 


many different fields. A. university research laboratory is also an 
ideal place in which to study both the physical and organic chemistry 
of the proteins and the natural tannins. Physical chemistry offers 
much the better prospects for immediate application to manufacturing 
practice, but both should be developed simultaneously. 

There is hardly any fundamental work in leather chemistry that 
is not suitable for the university laboratory, but, in order to com- 


mand the financial support of the industry, it must be done in such | 


a way as to make it directly serviceable to the industry. Appreciating 
the inertia that must be overcome before any great research move- 


ment can gain sufficient momentum to make it practically self-support- _ 
ing, the late Arthur H. Gallun, with remarkable foresight and lofti- 


ness of purpose, established a research in the fundamentals of leather 
chemistry, under the direction of Professor A. W. Thomas, of 
Columbia University, with the proviso that all results be published 
freely for the henefit of the industry as a whole. The results obtained 
during the past few years compare favorably with all previous work 
done on the mechanism of chrome and vegetable tanning. It will 
be evident from the description of this work, in the later chapters, 
that it must ultimately prove of great practical value and it is difficult 
to see how it can fail to gain the support of the entire industry in due 
time. It is worthy of special mention here as a demonstration of a 
kind of cooperation that should prove very profitable to both the 
university and the industry. 

It is hoped that the following pages will give chemists in many 
fields a better understanding of the problems of the leather industry 
and of the opportunities for cooperative research, and also give the 
industry itself a clearer appreciation of the possibilities for further 
extending the application of pure chemistry to leather manufacture. 


j 
i oe j 


@hapter 2} 
Histology of Skin. 


Since animal skin is the basis of leather, the importance of its 
histology to the science of leather manufacture israpparent.> =Never- 
theless scientific advancement, especially in regard to the preparation 
of skin for tanning, has been retarded by an insufficient knowledge of 
the histology of the skins of animals used in making leather. This 
has been due less, perhaps, to lack of appreciation of the value of 
histology than to the high degree of refinement of equipment and 
technic required for its study. 

Much of the complexity of the structure of the skin is due to the 
manifold purposes it serves. As a means of protection for the under- 
lying organs, it is so constructed as to act as a buffer against shocks 
or blows, while not interfering with the operation of any organs. 
It is an organ of sense, equipped with nerves sensitive to touch, pain, 
heat and cold, and, as an organ of secretion and excretion, it is supplied 
- with glands, ducts, muscles, and blood vessels. It serves also as a 
regulator of the body temperature, which it controls by regulating the 
evaporation of water from its surface and the secretion of oil to cover 
the surface in order to prevent too great a loss of heat. 

The degree to which each constituent part of the skin is developed 
depends upon the extent to which it is needed by the body and also 
upon the amount of available nourishment. The structure of a single 
skin varies considerably in different regions of the body. Nerve 
papilla, for example, are very numerous in regions where the sense 
of touch is most needed, as in the finger tips, and widely scattered 
in other regions. The skin structure is developed to meet sudden 
changes of temperature to the greatest extent in the regions of. the 
body most exposed to such changes and to resist friction and blows 
where these are most frequent. In fact, the skin tends to develop a 
structure at each point designed to be of greatest service to the 
body at that point. This results in a large number of types of skin 
structure that must be studied, depending upon the species of animal, 
the general nature of its feeding, the climatic conditions under which 
it lived, and the region of the body from which the specimen is taken. 

The number of possible types appears formidable, but tanners have 
learned from experience how to classify them in a general way accord- 
ing to the properties of the leather they yield. It seems reasonable 
to. believe that histologists may be able to develop a similar classifica- 
tion, based upon histology, that will prove extremely valuable in supple- 
menting the tanners’ information. 


15 


16 ‘THE CHEMISTRY OF LEATHER MANU FAG ee 


Because of the large number of highly trained investigators in the 
medical sciences, considerable progress has been made in the histology 
of human skin. This work is invaluable as a guide to the student of 
the histology of the skins of lower animals because most skins possess 
a common basic structure. But the several types of skin exhibit such 
marked differences in details of structure of vital importance in leather 
manufacture that a knowledge of the histology of a few specimens 
of human skin alone might actually be misleading. In order to apply 
histology intelligently to leather manufacture, separate studies must 
be made of the structure of each type encountered. 

Although much yet remains to be learned of the histology of skins 
used for leather manufacture, substantial progress has been made, 
the most notable work being that of Alfred Seymour-Jones.1 Sys- 
tematic studies have also been under way in the author’s laboratories, 
for several years, dealing with the structure of the skins of different 
animals and the changes which they undergo during the conversion 
of the skin into leather. In this book much of this work is presented 
for the first time. 


Preparation of Sections and Photomicrographs for Study. 


Since much of the information given in this chapter was obtained 
by direct observation of sections prepared in the author’s laboratories, 
a description of the methods employed will probably assist in making 
the presentation clearer, particularly so in view of the fact that work 
of this kind appears not to have been general in tannery laboratories. 
The description, however, will be limited to the methods used in the 
production only of the photomicrographs appearing in this book. The 
subjects of microscopy, microtomy, and photomicrography are too 
vast in scope for adequate treatment here and the reader desiring 
to pursue these subjects further is referred to the several excellent 
works available, such as those of Gage? and Lee.’ 

Sampling.—In studying the entire skin of an animal, strips of about 
0.5 x2 inches were cut from different parts of the skin so as ta 
show, not only the general structure, but also its variation throughout 
the skin. Care was taken, in cutting the strips, so that the later section- 
ing could be done in definite planes, as, for example, that including 
a hair follicle and erector pili muscle. It was found important that 
the plane selected be uniform for any given series of sections show- 
ing changes taking place during the passage of a skin through the 
tannery processes. 

Fixing.—After a tissue dies, the structure undergoes a gradual 
change unless it is immediately fixed. According to Lee, the word 
fixing implies two things: “first, the rapid killing of the element, 
so that it may not have time to change the form it had during life, 
but is fixed in death in the attitude it normally had during life; and 

1 Physiology of the Skin. Alfred Seymour-Jones. J. Soc. Leather Trades’ Chem. 
serially 1917-21. ‘ 

The Microscope. S. H. Gage. Comstock Publishing Co., Ithaca, N. Y 


* ee Microtomist’s Vade-Mecum, A. B. Lee. P. Blakiston’s’ Son & Co., Philadel- 
phia, Pa. 


HISTOLOGY OF SKIN 17 


second, the hardening of it to such a degree as may enable it to 
resist without further change of form the action of the reagents with 
which it may subsequently be treated. Without good fixation it ts 
impossible to get good stains or good sections, or preparations good 
in any way.” | 

The photomicrographs shown in this book are from sections fixed 
either in Erlicki’s fluid or in alcohol. Numerous other fixing agents 
were tried, but they did not answer so well for the specific purposes 
in view. E[rlicki’s fluid is made simply by dissolving 25 grams of 

potassium dichromate and 10 grams of copper sulfate in a liter of 
water. The strips of fresh skin were placed directly into this solu- 
tion without previous washing. ‘They were transferred to fresh solu- 
- tions daily for the first 3 days and then kept in the last bath until 
the solution had thoroughly penetrated them. The period of contact 
was usually from 5 to 7 days, after which they were washed in running 
tap water for about 20 hours and then dehydrated with alcohol. 

The skins of the sheep, cow, calf, and guinea pig, whose sections 
are shown in this book, were fixed immediately after the animals were 
killed. Their sections may, therefore, be regarded as showing the 
normal structure of the living skin. The other sections exhibit 
structures of skins as the tanner usually receives them. 

A duplicate series of strips of fresh skin was fixed in dilute alcohol, 
in each case, for comparison with those fixed in FErlicki’s fluid. Sec- 
tions from the Erlicki fixer generally showed various details more 
sharply than those from alcohol alone. All specimens of skin from 
the unhairing and bating processes were fixed in alcohol in order to 
avoid possible complications due to the reactions of the Erlicki fixer 
with the tannery liquors. Samples of air-dry leather were not fixed, 
but were imbedded in paraffine either directly or after soaking in santal- 
wood oil and then in molten paraffine. 

Dehydrating and Imbedding.—All specimens of skin, after fixing, 
were kept for the stated lengths of time in the following baths: 


6é 66 66 


t day 50 per cent alcohol 
“« “absolute alcohol 

fresh absolute alcohol 
Y% “  alcohol-xylene 

: carbol-xylene 

xylene 

fresh xylene 

molten paraffine. 


66 66 


The mixture of alcohol and xylene consisted of equal volumes of the 
two. The carbol-xylene is known as a clearing agent and has for its 
object the removal of alcohol from the specimen; it is prepared by 
mixing 25 cubic centimeters of melted phenol with 75 cubic centimeters 
of xylene. Very thick specimens had to be left in the molten paraffine 
for a much longer time, the object of this bath being to replace the 


1% THE CHEMISTRY OF LEATHER MANUFPAGEGRe 


xylene by paraffine. The strips from the paraffine bath were sus- 
pended in aluminum beakers, having a capacity of 100 cubic centi- 
meters, and covered with molten paraffine. The beakers were then 
plunged into cold water and kept there until the paraffine had com- 
pletely solidified. The beakers were then heated just sufficiently to 
loosen the paraffine blocks, which were pulled out and cut into the 
proper size and shape for placing in the microtome. 

Sectioning.—Really good work in preparing sections is possible 
only when the microtome knife is free from nicks and extremely sharp, 
the sharper the better. The thickness to which it is desirable to cut . 
the sections depends upon the particular part of the skin to be studied. 
For a general picture of the whole skin, a thickness of 20 microns is 
satisfactory. 

In the sections of skin taken from the unhairing processes, it will 
be noticed that there would be nothing to hold the loose epidermis 
and hair in place, if these were not securely fastened to the slide in 
some way. In order to prevent the loss of important material from 
the sections, the entire paraffine ribbons from the microtome were 
fastened to the slides by means of Mayer’s albumen fixative. This 
is made by mixing equal parts of glycerin and well-beaten white of 
egg, adding 2 per cent of sodium salicylate, and filtering. A tiny 
drop of this fixative was spread evenly over the middle of a slide 
with the finger and was then covered with water. A ribbon, con- 
taining a section of skin, was then floated onto the water, which was 
heated over an alcohol lamp carefully so as not to melt the paraffine. 
This causes the ribbon to spread out flat and it was then worked 
into place and smoothed out with a camel’s-hair brush. Slides prepared 
in this way were left to dry for at least one day, the sections 
meanwhile becoming securely fastened. They were then freed from 
paraffine by flooding the slides with xylene, after which they were. 
washed with absolute alcohol in preparation for staining. | 

Staining.—Six stains were used in preparing the sections shown in 
this book, with the exception of those of the human heel and scalp. 
These two sections were prepared in Professor Bast’s laboratory and 
were stained with Delafield’s hematoxylin and eosin. Where an 
aqueous stain was to be employed, the section was soaked, for sev- 
eral minutes, successively in the following strengths of alcohol: 95 per 
cent, 75 per cent, 50 per cent, 25 per cent, and then in water. After 
the staining, it was worked up through the series of solutions of alco- 
hol in the reverse order, finally being rinsed with absolute alcohol. 
The six stains used were prepared as follows: 

(1) Van Heurck’s logwood: 6 grams of powdered logwood ex- 
tract and 18 grams of alum were ground together in a mortar and 300 
cubic centimeters of water added slowly. The mixture was then filtered 
and 20 cubic centimeters of alcohol were added to the filtrate. The 
solution was kept exposed to air for several weeks, water being added 
to replace that lost by evaporation. The sections were kept in this 
stain for 3 minutes, rinsed in tap water until they turned blue, and 
then passed through the series of alcoholic solutions of increasing — 


fist OLOGY OFS KIN 19 


strength. Sections were transferred from the 95-per cent alcohol 
to the picro-indigo-carmine solution, where this was used for counter- 
staining. But where the counterstaining was done with bismarck brown, 
the sections were transferred from the 95-per cent alcohol to a 0.1-per 
cent solution of HCl in absolute alcohol, where they were kept until 
they turned pink and no more color was seen to wash away, after 
which they were rinsed with fresh alcohol and put into the bismarck 
brown stain. 

(2) Friedlander’s logwood: 2 grams of powdered logwood ex- 
tract dissolved in 100 cubic centimeters of alcohol were mixed with 2 
grams of alum dissolved in 100 cubic centimeters of water and 100 cubic 
centimeters of glycerin. This was used like Van Heurck’s stain. 

(3) Picro-indigo-carmine: To 100 cubic centimeters of 9o-per 
cent alcohol was added 1.0 cubic centimeter of absolute alcohol sat- 
urated with picric acid. This solution was then saturated with indigo 
carmine and allowed to stand with an excess of indigo carmine, with 
occasional shaking, for several weeks. The decanted solution was used. 
Sections were kept in this stain from 3 to 4 hours. 

(4) Picro-red: 5 cubic centimeters of absoltite alcohol saturated 
with picric acid were added to 55 cubic centimeters of go-per cent 
alcohol saturated with the dye Leather Red-X. This solution was 
diluted with alcohol to 10 times its volume before using. Sections re- 
mained in this stain for 2 minutes. 

(5) Weigert’s resorcin-fuchsin: 2 grams of basic fuchsin and 4 
grams of resorcin in 200 cubic centimeters of water were boiled for 
IO minutes, 25 cubic centimeters of a 30-per cent solution of ferric 
chloride were then added and the boiling was continued for 5 minutes. 
Then a saturated solution of ferric chloride was added until all of 
the color was precipitated. The mixture was allowed to stand over 
night to cool and settle and the supernatant liquor was decanted off 
and discarded. The residue was dissolved in 200 cubic centimeters of 
boiling 95-per cent alcohol and the hot solution was filtered into a 
bottle. After it had cooled, 5 cubic centimeters of concentrated HCl 
were added. For staining, this solution was diluted with an equal 
volume of alcohol and sections were left in it from 60 to 90 minutes, 
after which they were rinsed with alcohol. 

(6) Daub’s bismarck brown: To 95 cubic centimeters of absolute 
alcohol were added 5 cubic centimeters of saturated lime water and 
then more bismarck brown than would dissolve and the whole was 
shaken and allowed to settle, the solution being decanted off after 
standing for several days. Fifteen cubic centimeters of alcohol were 
added to the solution to replace any lost by evaporation, which would 
otherwise cause a precipitation of some of the dye. Sections were 
kept in this stain for 1 day. 

Mounting.—Since the sections of untanned skin were fastened per- 
manently to the slides before staining, the mounting of these was 
a very simple operation. After the sections were stained, they were 
rinsed successively with absolute alcohol, alcohol-xylene, carbol-xylene, 
and xylene. Each section was then covered with a drop of Canada 


20 THE CHEMISTRY OF LEATHER MANUPACI es 


balsam followed by a cover glass. Sections thus prepared are permanent 
and ready for study or photographing. 

Sections of leather, from the microtome, were uncurled on a piece 
of smooth paper and fastened by pressing on the paraffine surrounding 
the sections. They were then removed from the paper in a flattened 
condition by means of tweezers, dipped into a I-per cent solution of 
parlodion in equal volumes of alcohol and ether, and then transferred 
to slides previously coated with a thin film of santalwood oil. They 
were carefully smoothed out, covered with santalwood oil, and allowed 
to stand exposed to air until the alcohol and ether had evaporated, 
usually about 30 minutes. They were then washed with xylene and 
covered with balsam and cover glasses. As a rule the staining’ of 
leather sections for study is unnecessary, but a stain often assists in 
getting sharper photographs. Where a stain was employed on leather, 
the fact is noted under the photomicrograph. 

Photographing.—All photographs were taken with a standard type 
of photomicrographic apparatus. Wratten and Wainright “M” plates — 
were used and developed according to the directions which accompany 
them. The source of light transmitted through the sections was a 
6-volt mazda lamp having a concentrated filament. The light was 
filtered through appropriate color screens, consisting of the standard 
Wratten filters. The stains on the sections, together with the color 
screens, generally furnished all the detail or contrast necessary, but 
where this was not entirely satisfactory, the plates, after developing, 
were treated with standard intensifying or reducing agents as needed. 

Certain precautions were necessary in photographing grain surfaces 
for comparison. The hair follicles run obliquely to the surface, in 
consequence of which the lights and shadows depend upon the angle 
at which the light strikes the openings of the follicles. This was 
made uniform for all skins with nearly straight follicles by using for 
reference the plane including the line of the follicle and intersecting the 
plane of the grain at right angles. A beam from a powerful arc 
lying in the plane perpendicular to these two planes was made to strike 
the grain surface at an angle of 45 degrees. ; 

General.—Halftones were used to reproduce all photomicrographs 
excepting those in Figs. 20 to 27 inclusive, which were printed from 
line etchings. Because of the low magnification and consequent fine- 
ness of the fibrous tissues, a good reproduction could not be obtained 
by means of halftones; even a very fine screen produced an ap- 
preciable blurring effect. Line etchings, made without a screen, gave a 
much better result, the resulting increase in sharpness of the fine lines 
more than compensating for the loss in shading. 

All vertical sections are shown with the outer surface of the skin 
upward. Under each photomicrograph is given the location or region 
of the body from which the specimen was taken, where this was known, 
the thickness at which the section was cut in the microtome, the 
stains applied to the section, the eyepiece, objective, and filter used in 
photographing, and the final magnification of the section as it appears 
in the book. 


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Fig. 2.—Vertical Section of Human Skin. 
Location: lower part of back. 
Thickness of section: 20 wu. 
Stains: Van Heurck’s 

Daub’s bismarck brown. 


Eyepiece: none. 

Objective: 32-mm. 

Wratten filter: H-blue green. 
Magnification: 32 diameters. 


logwood, 


22 


HISTOLOGY OF SKIN | 28 


General Histology of Skin. 


It is not: uncommon to find in the literature descriptions of skin 
structure that apparently clash. Occasionally an author will present 
what purports to be a general description, but which is actually 
based upon the examination of a single type of skin structure. Figs. 1, 
2, and 3 all represent vertical sections of human skin, but the first 
was taken from the scalp, the second from the lower part of the back, 
and the third from the heel. A detailed description of one would 
give a very misleading picture of either of the others. In the section 
from the scalp, fat cells make up the greater portion of the whole, 
while in that from the back there are relatively very few fat ‘cells, 
but a great abundance of fibers of connective tissue. In the section 
from the heel, fat cells and connective tissues are both very prominent, 
but no hairs are seen. Practically, these sections represent very differ- 
ent types, yet all three conform to a common basic structure. A 
structure common to all skin may be greatly exaggerated in one type 
and scarcely detectable in another. 

All four general classes of tissues, epithelial, connective, muscular, 
and nervous, are present in the skin, as well as those of the blood. 
These tissues either consist of cells or are the product of cells. The 
epithelial tissues consist of layers of cells, which cover all the free 
surfaces of the animal body. The connective tissues are distinguished 
from the other fundamental tissues of the body by the fact that 
their cells lie imbedded in extracellular material which appears to 
be the result of their activity. The various types of connective tissues 
are distinguished among themselves by the kind of extracellular tissue 
which they produce, such as bone, cartilage, etc. The muscular tissues 
have a well developed power of contracting, apparently without change 
of volume, the decrease in length being compensated by an increase 
‘n diameter. The cells of the striated or involuntary muscles are long 
in relation to their width and are marked with transverse bands, while 
those of the nonstriated or involuntary muscles are spindle shaped, 
without transverse striations. The nervous tissue found in the skin 
is a protoplasmic prolongation of cells lying in the central nervous 
system, or in the ganglia closely associated with that system. 

The skin is divided sharply into two layers, distinct both in 
structure and origin: a relatively very thin outer layer of epithelial 
tissue, the epidermis, and a much thicker layer of connective and other 
tissues, the derma. Raw skin, as an article of commerce, has also a 
third layer, the superficial fascia, known to the tanner as the adipose 
tissue or, more commonly, the flesh. In keeping with the nomenclature 
of the leather trade, the word flesh will be used only in this con- 
nection, although in anatomy flesh really means muscle tissue. In 
life, the adipose tissue, or flesh, connects the skin proper very loosely 
to the underlying parts of the body. The derma lies between the 
epidermis and adipose tissue. 

In the preparation of skin for tanning, except in special cases, 
such as the tanning of fur skins, the adipose tissue and the entire 


Fig. 3.—Vertical Section of Human Skin. 


Location: heel. Eyepiece: none. 

Thickness of section: 30 w. Objective: 48-mm. 

Stains: Delafield’s hematoxylin, Wratten filter: H-blue green. 
Eosin. Magnification: 20 diameters. 


24 


Hist OLOGY OF SKIN 25 


epidermal system must be removed intelligently and with extreme care, 
leaving the derma to be converted into leather. The epidermal system, 
adipose tissue, and derma will be described in turn. 

The epidermis is made up of a cellular strata originating from the 
ectoderm, the outer layer of the young embryo, and independently 
of the derma, which is derived from the mesoderm, or middle layer. 
These two layers grow independently throughout life and differ ma- 
terially in both chemical and physical properties. In Fig. 2 the epi- 
dermis can be seen as a dark band forming the upper boundary of 
the skin and constituting only about 1 per cent of the total thickness. 
So far as its growth is concerned, the epidermis may be looked upon 
as a parasite, although it is a most important part of the body. It 
has no blood vessels of its own, but rests upon the upper surface of 
the derma and draws its nourishment from blood and lymph supplied 
by the blood vessels of the derma. It grows only through the reproduc- 
tion of its own cells. 

The portion of epidermis in contact with the derma is a layer of 
living epithelial cells, rather elongated in shape. It may be men- 
tioned that a cell consists of a nucleus suspended in protoplasm en- 
closed between very thin walls acting as a semi-permeable membrane. 
Nourishment from the lymph and blood streams diffuses through the 
cell walls, and after a certain period of growth the cell divides 
mitotically, forming two cells. ‘This change appears to be initiated 
by the nucleus. In the deepest layer of the epidermis, each cell increases 
in height and then subdivides, forming two cells, one above the other. 
_ This process is repeated indefinitely. As the older cells are pushed 
outward, they become flattened by dehydration and other changes. 
During this process, the protoplasm dries up and the cells lose their 
power of reproduction. In the outermost layer, the cells are very dry 
and scaly and are gradually worn away. ‘This scaling is often very 
noticeable on the scalp in the form of dandruff, which, in itself, is 
not the result of a disease, but rather is evidence that the epidermal 
cells are functioning and reproducing vigorously. 

Where the epidermis is very thick, as on the heel, the gradual 
transition which the cells undergo in their outward course gives the 
epidermis the appearance of having several distinct layers. The por- 
tion of the epidermis shown in the upper left hand corner of Fig. 3 
is shown at a very much higher magnification in Fig. 4. Now the several 
strata can be seen very plainly. 

The layer marked E is the uppermost part of the derma and 
numerous protuberances of its surface, called papillae, can be seen 
extending upward into the epidermis, giving the boundary between 
epidermis and derma a serrated appearance. D is the Malpighian layer 
of the epidermis, or stratum mucosum. It is built up of several rows 
of living epithelial cells, whose nuclei appear in the picture as dark 
spots or rods. ‘Tiny fibers, often called prickles, pass from cell to 
cell, holding them together and securing them to the derma. Extending 
between these prickles, which look as if they were walls in section, 
are protoplasmic processes and it is supposed that food passes upward 


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HISTOLOGY OF SKIN 27 


between the cells and waste from the upper layers downward. From 
this food the cells derive the nourishment necessary for reproduction. 
This layer contains no blood vessels, but very fine nerve fibers pass 
from the derma into this layer, forming a network between the cells 
and terminating in bulbous swellings or undergoing a gradual breaking 
up into nerve granules. 

As the new cells are formed, the older ones are pushed outward 
where nourishment is no longer available and the protoplasm of the 
cells gradually dries up. Upon staining, the cells then appear as 
though they contained coarse granules and form the layer shown at C, 
which, from its appearance, has been called the stratum granulosum. 
The cells also contain a pigment, which is at least partly responsible 
for the color of the skin. This pigment, known as melanin, is thought 
to be a derivative of hematin containing iron and sulfur. It is very 
concentrated in the skin of the negro and almost entirely absent from 
the skin of a blonde. Apparently the pigment is formed as a pro- 
tection against strong sunlight, both for the skin and the underlying 
tissues. The pigmented layer may thus be looked upon as a color filter. 
When the pigment-containing cells are collected in spots, they appear 
as freckles. The pigment in the negro skin is found in the deepest 
cells of the stratum mucosum, in the connective tissue cells of the upper 
part of the derma, and in the wandering cells of the lymph, found in 
the lymph spaces or between the cells of the epidermis or connective 
tissues. The pigment granules are found only in cells. 

As the cells are pushed still further outward, the cell granules 
break down, yielding a material, called eleidin, which resists staining 
and gives the epidermis in this region a transparent appearance, from 
which it has derived the name stratum lucidum. This layer is shown 
at B. 

The cells continue to undergo changes during their outward course, 
becoming drier and flatter, and finally form the very thick layer shown 
at A, the stratum corneum, in which the cells tend to break away from 
each other and to scale off. This layer is being worn away continually 
and is replaced by the newer cells from below. The corneous layer 
is a very poor conductor of heat and the waxy material usually present 
on its surface makes it water repellent. In the photomicrograph a duct 
can be seen taking a spiral course up through the corneous layer. This 
is the outlet of a sudoriferous or sweat gland seated in the derma. 
Its opening at the surface of the corneous layer is called a pore. 

All of the strata noted above can be detected only where the 
epidermis is very thick. Elsewhere only the stratum mucosum and 
stratum corneum are visible. In no case have we yet observed a sec- 
tion of skin used for making leather where more than these two layers 
could be recognized in the epidermis. 

The independent growth of the epidermis and derma involves a 
number of important appendages of the skin. In the epidermal sys- 
tem, the reproduction of epithelial cells produces, not only the epi- 
dermis, but also the hair and the sebaceous and sudoriferous glands. 
These cellular structures are composed of proteins of the class known 


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HISTOLOGY OF SKIN 29 


as keratins as distinct from the collagens and elastins of the derma. 
Where a portion of the epidermis is lost, through accident, it can 
be regenerated only by the surrounding epithelial cells spreading over 
the bare spot, by reproduction. The necessity for removing the 
epidermal system completely before tanning and without any injury 
to the derma makes the difference in chemical composition between the 
two systems a matter of great importance to the tanner. 

In the class with hair belong also nails, claws, hoofs, scales, and 
feathers, which are all special growths of the epidermis. To the 
naked eye, the hair appears to pierce the skin, but actually it does not 
do so. An examination of Fig. 2 will show that the epidermis dips 
down into the body of the derma, forming a pocket, or follicle, in which 
the hair grows. The follicle is complex in structure because it is 
made up of the epidermal layers on the hair side and of the layers 
of the derma on the other. At its bottom, the follicle is penetrated 
by a projection coming from the derma and known as the hair papilla, 
which is supplied with both nerves and blood vessels. 

A good example of a hair papilla is shown in Fig. 5 in the hair 
bulb from the skin of a hog. The bottom end of the bulb appears like 
a pair of pincers with the jaws slightly open and facing downward. 
A similar structure may be seen in the hair bulbs of the scalp shown 
in Fig. 1. Passing through the opening in the jaws into the large 
open space above and resembling a candle flame in shape is the papilla, 
which contains tiny nerves and blood vessels which supply nourish- 
ment. Lining the lymph space surrounding the papilla are numerous 
epithelial cells, which derive from the blood and lymph the nourish- 
ment necessary for reproduction. As new cells are formed, the older 
ones are pushed outward through the follicle, forming the hair. The 
rate of growth of the hair is determined by the rate at which the cells 
surrounding the papilla reproduce. 

The newly formed cells of the hair, like those of the Malpighian 
layer of the epidermis, are very soft. As they are pushed upward, 
they become elongated in shape and harder. In forming the hair, they 
assume the shape of the follicle; if this happens to be curved, the 
hair will be curly. In the negro, the follicles often have a curvature 
of nearly go degrees, which accounts for the tightness of the curls. 

The portion of the hair showing above the surface of the skin 
is called the shaft and the lower portion the root, which enlarges 
‘nto a bulb at its lower extremity, where it is penetrated by the hair 
papilla. The shaft is made up of a central medulla, or pith, of rounded 
cells, containing eleidin granules, surrounded by a much thicker por- 
tion composed of long fibrillated cells, containing pigment, and en- 
closed by an outer layer of cells which become hardened in the form 
of overlapping scales. These scales, which give fur and wool their 
felting properties, open outward so as to resist the pulling out of the 
hair. Unless the lighting is properly adjusted and the magnification 
sufficiently great, the scales are not easily discernible. In Fig. 6 may 
be seen the scales of a tiny piece of wool. The scales of one side 
and the shadows of those on the other both show because the wool 


ing oot ne: Silane ed nanmrpti be gaat,» As TT BE octet A 


Fig. 6—Segment of Sheep Wool. 
Stain: none. Wratten filter: H-blue green, 


Eyepiece: -7.5X. Magnification: 1260 diameters. 
Objective: 4-mm. 


30 


Pie OLOGY CGP SKIN es 


was photographed with transmitted light. The same general structure 
can be seen on most hair, but it is not always so pronounced. . 

When a hair is shed, after reaching the limit of its existence, 
the epithelial cells left surrounding the hair papilla keep on multiplying 
and soon another hair is formed to replace the one shed. Baldness 
results from the failure of the blood vessels of the papilla to furnish 
the required nourishment or from the destruction of the epithelial 
cells in some other way. Any serious attempt to grow hair on a bald 
head must be accompanied by some means of introducing living epi- 
thelial cells into the hair follicles, of which there are something like a 
thousand to the square inch. In other words, we cannot grow a crop 
without seeds or seedlings. 

In old age, pigment is no longer available for the hair cells and 
the new hairs, containing no pigment, appear gray in color. Hair 
containing pigment, however, may look white by reflected light, due 
to the presence of tiny air bubbles among the cells. 

Each hair follicle is supplied with sebaceous glands with ducts 
emptying into the upper portion of the follicle. A group of these 
glands can be seen in Fig. 2. They are lined with epithelial cells 
which secrete from the blood the materials required for the synthesis 
of the oils which they produce. When they become charged with 
oil, the protoplasm disappears and the cell breaks down, discharging 
the oil into the duct. New cells are continually being formed to re- 
place the old ones. The oil is forced into the follicle, where it coats 
and lubricates the hair, and finally to the surface of the skin, which 
it softens and protects against the cold. In contact with air, this: oil 
thickens to the consistency of ear wax, to which it is related. When 
the ducts become clogged with dirt, the pressure behind them causes 
them to become distended, giving rise to blackheads. Sebaceous glands 
are sometimes found also in parts of the skin free from hair. 

Attached to each hair follicle, just below the sebaceous glands, 
and extending obliquely upward through the derma, almost to the sur- 
face, is a bundle of nonstriated muscle tissue, known as the erector 
pili muscle. In Fig. 2 one of these muscles forms a V with the hair 
follicle, and the sebaceous glands may be seen within the angle so 
formed. The nerves supplying these muscles are known as the pilo- 
motor nerves. These muscles contract under the influence of emotions, 
such as fear, surprise, anger, or other disagreeable states, or in re- 
sponse to cold or grazing tactile stimuli. Among the commoner visible 
effects are the roughening of the skin called goose-flesh and the 
effect of the hair standing on end, very pronounced in a frightened 
cat. 

_ The real purpose of the erector pili muscles 1s apparently to pro- 
tect the body against sudden changes of temperature by their con- 
trol over the operation of the glands; they seem to act as effectively 
as a thermocouple in a good thermostat. Their contraction puts a pres- 
sure on the glands which causes the cells to give up their oil to the 
hair follicle and, in the process, the cells are destroyed. The oil is 
then forced up through the follicle to the surface of the skin, where 


Fig 7.—Vertical Section of Calf Adipose Tissue. 


Location: butt. 
Thickness of section: 20 uw. 


Stains: Van Heurck’s 
Daub’s bismarck brown. 


logwood, 


32 


Eyepiece: none. 

Objective: 16-mm. 

Wratten filter: H-blue green. 
Magnification: 70 diameters. 


al lle tne ie 


me OLOGY Ot SKIN 33 


it tends to stop the action of the sudoriferous glands and _ the 
evaporation of water from the surface of the skin. 

The sudoriferous or sweat glands are coiled sacs with spiral ducts 
leading to the surface of the skin. In Fig. 3 several of these ducts 
can be seen winding up through the epidermis and terminating at 
the surface as pores. Often the ducts seem to lead into the hair 
follicles above the openings of the ducts of the sebaceous glands. 
The sacs of the sweat glands are lined with epithelial cells, which are 
continuous with the cells of the Malpighian layer of the epidermis, 
and which secrete water, salts, urea, and other wastes from the blood 
and pass them out through the ducts. Where no sebaceous glands are 
present, the sudoriferous glands also provide an oily fluid to keep 
the surface of the skin soft. These glands serve the dual purpose 
of disposing of waste products and of permitting control of the body 
temperature through the regulation of the rate of evaporation of 
water. ; 

This entire epidermal system, including the epidermis, hair, and 
sebaceous and sudoriferous glands, must be removed from the skin in 
such manner that the derma suffers no injury that can be detected 
in the finished leather. 

The skin is connected to the underlying parts of the body very 
loosely by means of fibers of connective tissue, usually called adipose 
tissue because it is so frequently the seat of fat deposits, most 
numerous in the vicinity of the abdomen, which serve to protect the 
body against cold. The looseness of connection allows the skin very 
free movement and, incidentally, makes flaying a much simpler matter 
than it would otherwise be. ‘The adipose tissue, while not a part of 
the skin proper, is of importance to the tanner because much of it 
remains adhering to the skins received at the tannery and must be 
removed prior to tanning. If left on the skins, it greatly impedes the 
progress of tanning. 

In Fig. 7 is shown a vertical section of adipose tissue from the 
butt of a calf skin along with the lower portion of the derma. The top 
quarter of the picture shows a portion of derma bound on its under 
side by strands of elastin fibers, appearing as compact masses of black 
threads; actually they are of a pale yellow color. The fat cells of 
the adipose tissue are arranged in layers and are held together by 
fibers of connective tissue. The light colored tissues are the white 
fibers, composed of collagen, and the dark ones are the yellow fibers 
of elastin. Large arteries, nerves, and veins which supply the derma 
traverse the adipose tissue in many places and can often be seen heavily 
protected with connective tissue. ‘This region is sometimes supplied 
also with striated muscle fibers to permit the voluntary twitching of 
the skin. 

The removal of the adipose tissue of the skin, preparatory to 
tanning, is an operation known as fleshing. This is done efficiently 
when all of the tissues underlying the derma are cut away, leaving the 
derma itself entirely intact. 

It is the derma, or true skin, that is actually used to make leather 


Fig. 8.—Vertical Section of Reticular Layer of Calf Skin. 


Location: butt. 


Thickness of section: 20 wp. 


Stains: Van Heurck’s 
Daub’s bismarck brown. 


logwood, 


34 


Eyepiece: 5X. 

Objective: 16-mm, 

Wratten filter: B-green. 
Magnification: 170 diameters. 


Brew LOGY (OF SKIN 35 


and the chief leather-forming constituent of the derma is collagen, 
the substance of the white fibers of connective tissue. Sound leather 
can be produced only from skins in which these fibers are well de- 
veloped and abundant. The three contrasting structures in Figs. 1, 
2, and 3 are typical of the extremes found in the skins of the lower 
animals. A skin composed chiefly of fat cells is of little value in making 
leather and one in which large groups of fat cells are interspersed 
between the collagen fibers will yield only a spongy leather because 
of the empty spaces left after the fat cells have been destroyed in 
the processes preparatory to tanning. The tendency toward one ex- 
treme or the other depends largely upon the habits and feeding of 
_the animal as well as upon its species. In considering the general 
structure of skin, one should look upon the major portion of the 
derma as consisting of both fat cells and connective tissues, either 
of which may be very abundant or relatively scarce. 

Unlike the epithelial tissues, the major portion of the connective 
tissues is not made up of cells, but results from the activity of migra- 
tory cells very much smaller in size than the extracellular material. 
The relation of these cells to the collagen fibers of calf skin can be 
seen in Fig. 8. The cells stain more deeply than the fibers and appear 
in the picture as black specks having a diameter of about 1 millimeter, 
which means that the actual cells have a diameter of about 1/170th 
of this. In the sections we have examined, the abundance of these 
cells diminishes with increasing age of the animal. 

By examining the cross sections of fibers running pefpendicular 
to the plane of the page, the arrangement of the fibers, or fibrils, in 
bundles can be seen very plainly. Seymour-Jones regards the fibers 
as enclosed in very thin sheaths of what he terms “fiber sarcolemma.” 
‘While we have not been able, as yet, to detect such a sheath micro- 
scopically, the investigations of Wilson and Gallun, described in Chap- 
ter 8, seem to indicate that the surfaces of the collagen fibers are very 
much more resistant to tryptic digestion than the material just under 
the surface. 

Of the two kinds of fibers composing the connective tissues, the 
collagen fibers are very much thicker and more abundant than the 
elastin fibers. ‘There is usually a dense layer of elastin fibers at the 
lower surface of the derma, where it is attached to the adipose tissue, 
as shown in Fig. 7, and another in the region of the erector pili muscles. 
But the greater portion of the derma seems to contain relatively few 
elastin fibers and these are generally to be found surrounding the 
blood vessels and nerves traversing the derma. 

The main trunk lines of blood vessels and nerves supplying the 
derma run parallel to the surface just above the lower elastin layer. 
From these trunk lines branches shoot upward and are distributed to 
all parts of the derma. A network of lymph ducts also is distributed 
throughout the skin. 

Cross sections of the arteries and veins show three distinct layers: 
an outer layer of collagen and elastin fibers, a middle layer of non- 
striated muscle tissue and elastin fibers, and an inner membrane of 


Fig. 9.—Grain Surfaces of Tanned Skins. - 


Wratten filter: K2-yellow. 
Magnification: 7 diameters. 


Eyepiece: none. 
Objective: 48-mm. 


36 


Vin ea 
Horned Toad 


Fig. 10.—Grain Surfaces of Tanned Skins. 


Eyepiece: none. - Wratten filter: K2-yellow. 
Objective: 48-mm. Magnification: 7 diameters. 


of 


clonal owe: CHEMISTRY OF LEATHER MANUFACTURE 


flattened cells. All three layers are pronounced in the arteries, but in 
the veins the outer layer is very much thicker than the inner layers, 
which are much less developed and collapse when the vein is empty. 
The veins are also equipped with semilunar valves in order to prevent 
backflows of blood. 

A cross section of an artery can be seen at the top (ot gHig.n7 
It is the large circular body just to the left of the midline. To the 
right of the artery is a vein, which has collapsed. The circular mass 
just under the artery is a cross section of a bundle of nerves. Three 
more sections of nerve bundles are prominent, elongated in shape, 
two just below the vein and one to the extreme left of the artery. 

In those parts of the body where the sense of touch is well de- 
veloped, as in the fingers, there are numerous protuberances of the sur- 
face of the derma into the epidermis, called papille. These are very 
pronounced in the section of skin from the human heel shown in Fig. 3. 
They are arranged in definite patterns which do not change throughout 
life. The design of the thumb print is produced by the papillae. They 
seem to be absent entirely from some parts of the body, particularly 
where the sense of touch is not well developed and where the epidermis 
is very thin. They are of two kinds, one containing blood vessels 
furnishing-lymph to the active epithelial cells in their vicinity and the 
other containing the nerves sensitive to touch, pain, heat and cold. The 
epidermis above the papillz is thinner than at other points, the papillze 
serving the purpose of bringing the nerve ends nearer to those surfaces 
where they are most needed. 

The portion of the derma immediately in contact with the epidermis 
has been called the “grain membrane” by Seymour-Jones because it 
forms the grain surface of the finished leather. Although its boundary 
on the side in contact with the epidermis is very sharp, on the other 
side it blends into the rest of the derma with no sharp change of 
properties. The fibers of connective tissue grow finer as they near 
the grain surface, in which the fibers are extremely fine and generally 
run parallel to the surface. They can be seen very plainly in the hori- 
zontal section of tanned calf skin shown in Fig. 150, of Chapter 16. 
Whether or not the fibers of the grain surface are continuous with 
those of the connective tissues of the derma, they seem to possess 
somewhat different properties. When unhaired skin is kept in boiling 


water, the fibers of the grain surface remain as a thin sheet, although - 


somewhat changed, long after the larger collagen fibers below have 
passed into solution as gelatin. The outer surface is then very sharp, 
but the inner side, facing the remnants of the collagen fibers, appears 
jellylike and heterogeneous, indicating a gradual change in properties 
of the fibers as they pass from the derma into the grain surface. ; 

It is of great importance that no damage be done to the grain 
surface in removing the epidermis, because it determines the appear- 
ance of the finished leather. It is therefore fortunate for the tanner 
that the fibers in this surface are more resistant to the action of 
alkalies than the epidermis above it and more resistant to the action of 
tryptic enzymes than the elastin fibers below it. The grain surface 


ee a ee 


at a 


WISHOLOGY OF SKIN 39 


is readily attacked by proteolytic bacteria under certain conditions, 
however, resulting in what is known to the tanner as pitted grain. 

The design of the grain surface, as seen on the skin after unhairing 
and tanning, is distinct for each species of animal, while the fineness 
of the pattern is an indication of the age of the animal. It is due 
to the arrangement of the hair follicles and pores, and of the papillz 
where these are present. The grain surfaces of the tanned skins of 
a number of different animals are shown in Figs. 9 and 10. They 
are all magnified to exactly the same extent and are directly com- 
parable. It will be noted that the cow and calf have the same pattern, 
but that it is much coarser in the older animal. These designs can be 
used to identify different species of animal. 

We shall now turn from considering the general histology of skin 
to the more detailed structures shown by definite types of skins used 
in making leather. 


Cow Hide. 


In selecting skin for the production of heavy, sound and durable 
leather, the tanner usually chooses the hide of the steer or cow. In 
Fig. 11 is shown a vertical section of cow hide taken from the thickest 
part of the butt. The specimen was fixed in Erlicki’s fluid immediately 
after the death of the animal. This is the type of skin suitable for 
manufacture into sole leather or heavy belting or harness leather. Over 
80 per cent of the total thickness of. the hide is made up of heavy, 
interlacing bundles of collagen fibers, the chief leather-forming con- 
stituent of skin, and very few of the fat cells that tend to make the 
leather spongy are to be found among these fibers. 

The epidermis appears as a thin, dark line forming the upper 
boundary of the section and occupying barely one-half of one per cent 
of the total thickness, the rest being the derma, the adipose tissue having 
been removed from this portion of the hide in flaying. The epidermis 
can be seen to dip down into the derma in many places, forming the 
follicles in which the hairs grow. 

The presence of the muscles, glands and follicles in the top fifth 
of the derma give this region the appearance of a layer quite distinct 
from the lower part of the derma. Indeed, it is advantageous, in 
leather manufacture, to look upon the derma as divided into two 
distinct layers. The dividing line might conveniently be taken as that 
formed by the deepest points of the sudoriferous, or sweat, glands. 
The lower, fibrous region of the skin is often referred to as the 
reticular layer because of the network appearance of the collagen fibers. 
This name might well be accepted for most skins suitable for leather 
manufacture, although it might seem somewhat strained for skins 
in which the derma is made up largely of fat cells. The chief func-— 
tion of the upper layer seems to be that of a thermostat for the body 
and the writer, therefore, proposes the name thermostat layer as 
indicating its structure as well as its chief function. 

In Fig. 11 the thermostat layer occupies the top fifth and the 


Fig. 11.—Vertical Section of Cow Hide. 


Location: butt. Eyepiece: none. 

Thickness of section: 20 u. Objective: 48-mm. 

Stains: Van Heurck’s logwood, Wratten filter: F-red. 
Daub’s bismarck brown. Magnification: 19 diameters. 


40 


Fig. 12.—Vertical Section of Thermostat Layer of Cow Hide. 


Location: butt. Eyepiece: 5X. 

Thickness of section: 20 uw. Objective: 16-mm. 

Stains: Van Heurck’s logwood, Wratten filter: H-blue green. 
Daub’s bismarck brown. Magnification: 85 diameters. 


4I 


42 THE CHEMISTRY OF LEATHER MANUFAGEORe 


reticular layer the remaining four-fifths of the section. The advantage 
of dealing with these layers separately is made clear by the fact that 
the structure of the reticular layer determines the physical properties 
of the leather such as tensile strength, solidity, resilience, etc., while 
the thermostat layer determines more particularly the appearance of 
the leather. In making the finer grades of leather, a great deal of 
attention must be paid to the thermostat layer. It is a matter of 
considerable importance that this layer is almost as thick in a small 
skin as in a large one; in the thinner skins and even in the thinner 
parts of the same skin, this layer occupies a greater proportion of the 
total thickness. 

The section in Fig. 11 is magnified only 19 diameters. In order 
to show the structure of the thermostat layer in greater detail, the upper 
left hand corner of this section was magnified to 85 diameters. At 
this greater magnification, it is shown in Fig. 12. The Malpighian and 
corneous layers of the epidermis can now be clearly differentiated, 
the latter becoming extremely thin where it lines the hair follicle. 
The stratum granulosum and stratum lucidum do not appear to be 
present in the epidermis. Attached to the base of the hair follicle 
and weaving its way upward to the right is the erector pili muscle. 
Just above this muscle and emptying into the hair follicle is a group 
of sebaceous glands. The empty space near the lower left hand corner 
is that formerly occupied by a sweat gland whose duct has wandered 
out of the plane of the section, reappearing as a pore to the right of 
the hair just at the entrance to the hair follicle. The fine, black, 
threadlike lines running roughly parallel to the surface and to be 
found throughout the thermostat layer are the elastin fibers, or yellow 
fibers of connective tissue. In this layer, the collagen fibers are very 
much finer than in the reticular layer and appear to be broken up 
into individual fibrils. The grain surface appears only as portions 
of tiny fibrils with no sharp line of division from the rest of the derma. 
No papillz are to be seen in this section; in fact, we found no papillz 
in any part of the cow hide, except in the region of the legs. 

In order to present a still clearer picture of the important thermo- 
stat layer, we prepared series of sections parallel to the surface of 
the hide. Strips of hide imbedded in paraffine were placed in the 
microtome and sections, each 20 microns thick, were cut in succession 
from the corneous layer to a point in the reticular layer, every section 
being kept in order and mounted. The five horizontal sections shown 
in Figs. 13 to 17 were prepared from a strip of hide taken from 
the thigh so as to include the papillz, which were not present in the 
other regions. Fig. 13 is a section cut through the epidermis. In the 


center is the opening of a hair follicle. The circular mass just above - 


the center is the cross section of a hair. The stringy lines forming 
an oval shaped mass about the hair are the part of the corneous layer 
of the epidermis which dips down into the hair follicle. The heavy 
dots seen throughout the rest of the picture are the nuclei of the 
cells of the Malpighian layer of the epidermis. The irregularly shaped, 
light-colored patches are cross sections of the papille of the derma 


HISTOLOGY OF SKIN 43 


Fig 13.—Horizontal Section of Cow Hide. 
(Through epidermis.) 


Location: thigh. Eyepiece: 5X. 

Thickness of section: 20 uw. Objective: 8-mm. 

Stains: Van MHeurck’s logwood, Wratten filter: H-blue green. 
Daub’s bismarck brown. Magnification: 200 diameters. 


which protrude into the epidermis and are made up chiefly of nerves 
and blood vessels. | | 

Fig. 14 represents a section cut 0.30 millimeter below the upper 
surface of the corneous layer. It marks the plane of the derma where 
the ducts of the sebaceous glands empty into the hair follicles. In 


44 THE CHEMISTRY OF LEATHER MANUFACTURE 


Fig. 13.—Horizontal Section of Cow Hide. 
(0.30 mm. below upper surface.) 


Location: thigh. Eyepiece: 5X. ; 

Thickness of section: 20 wu. Objective: 8-mm. 

Stains: Van MHeurck’s logwood, Wratten filter: H-blue green. 
Daub’s bismarck brown. Magnification: 200 diameters. 


the lower part of the middle of the picture can be seen the cross sec- 
tion of a hair and of two ducts emptying into the follicle, just above 
the hair, to right and left. Both the ducts and the follicle are lined 
with epithelial cells which are continuous with the Malpighian layer 
of the epidermis and of which they are appendages. The dark, thread- 


HISTOLOGY OF SKIN 4S 


Fig. 15.—Horizontal Section of Cow Hide. 
(0.54 mm. below upper surface. ) 


Location: thigh. Eyepiece: 5X. 

Thickness of section: 20 Objective: 8-mm. 

Stains: Van Heurck’s logwood, Wratten filter: H-blue green. 
Daub’s bismarck brown. Magnification: 200 diameters. 


like structures are elastin fibers. The tiny collagen fibers of this region, 
being stained more lightly, are not prominent. 

The section in Fig. 15 forms the plane 0.24 millimeter below that 
of Fig. 14. The hair whose cross section is shown in the lower part 
of the middle of Fig. 15 is the same as that shown ite bigs dee Loe 


46 THE CHEMISTRY OF LEATHER MANUFACTURE 


Fig. 16.—Horizontal Section of Cow Hide. 
(0.54 mm. below upper surface. ) 


Location: thigh. Eyepiece: none. 


Thickness ot section: 20 ,. Objective: 16-mm. 
Stains: Van Heurck’s logwood, Wratten filter: H-blue green. 
Daub’s bismarck brown. Magnification: 48 diameters. 


hair follicle at this point has a much thicker wall of epithelial tissues 
and is more thickly bound by elastin fibers. Above the follicle, to the 
right and left, are the two groups of sebaceous glands whose ducts 
can be seen emptying into the follicle in Fig. 14. These glands re- 
semble bunches of grapes. Each dot is a cell nucleus and the fine 


HISTOLOGY OF SKIN 47 


a 
oe 

» 

ee 


Ge es a , 
LI OM CM A IS 


Fig. 17.—Horizontal Section of Cow Hide. 
(0.84 mm. below upper surface.) 


Location: thigh. Eyepiece: 5X. 

Thickness of section: 20 wn. Objective: 8-mm. 

Stains: Van Heurck’s logwood, Wratten filter: H-blue green. 
Daub’s bismarck brown. Magnification: 200 diameters. 


lines are the thin walls bounding the cells. A portion of the erector 
pili muscle is visible at the midpoint of the top of the picture. It 
is passing obliquely upward through the plane of the section and away 
from the hair follicle. The contraction of this muscle exerts a pressure 
upon the cells and their oily contents are forced up through the ducts 


48 THE CHEMISTRY OF LEATHER MANUFACTURE 


and into the hair follicles at the openings shown in Fig. 14. Between 
the two groups of glands and the hair follicles is a mass of muscle 
tissue of the same kind as that constituting the erector pili muscle. 
Apparently the muscle extends also into this region and exerts its 
pressure upon the cells by a sort of pinching action. 

Fig. 16 is a photomicrograph of this section taken at lower mag- 
nification so as to show the general arrangement of follicles and glands. 
The portion appearing in Fig. 15 can now be recognized just below 
the center of the picture. Associated with the hair we have been fol- 
lowing are three others, and this tendency for the hairs to group 
themselves in threes and fours is very noticeable. Some of the follicles 
are not so deeply seated as others and have their sebaceous glands in 
a plane higher up. This explains why no glands are to be seen in 
the vicinity of some of the follicles. The short, thick lines appearing 
here and there are arteries or veins wandering in and out of the plane 
of the section. 

In Fig. 17 is shown the section forming the plane 0.30 millimeter 
below that of Fig. 15, or a total distance of 0.84 millimeter from the 
upper surface of the corneous layer. A cross section of the same 
hair as that shown in Figs. 14 and 15 appears in the center of the 
picture, but this time we have cut right through the hair bulb. The 
black mass is the bulb and the light patch at its center is the hair 
papilla. To the right and left and above the hair bulb are the sweat 
glands. They appear as large, empty sacs, with portions of their 
linings of epithelial cells showing like leopard spots. In this plane 
the elastin fibers are much less numerous than in the regions higher 
up and the collagen fibers are now much larger and grouped in bundles. 
At a distance of 0.12 millimeter below this plane, we encounter the 
last of the epithelial cells of the sweat glands and therefore the lower 
boundary of the thermostat layer. : 

The reticular layer consisted almost entirely of collagen fibers, 
elastin fibers being present only in the lowest region and surrounding 
the blood vessels and nerves traversing other parts of the reticular 
layer. 


Calf Skin. 


A calf skin, very naturally, appears much like a cow hide in minia- 
ture. In Fig. 18 is shown a vertical section from the skin of a healthy 
young heifer calf, which had been fixed in Erlicki’s fluid immediately 
following the slaughter and flaying of the animal. As a rule, the skin 
of a heifer calf has greater solidity and fineness of appearance than 
that of a steer calf and is, consequently, to be preferred for leather 
making. In comparing Figs. 11 and 18, it should not be overlooked 
that the section of calf skin is magnified more than twice as highly 
as that of the cow hide. In fact, in making comparisons of any 
photomicrographs in the book, erroneous conclusions may be drawn, 
if the magnifications are not taken into consideration. 

The relatively greater thickness of the thermostat layer in the calf 


Fig. 18.—Vertical Section of Calf Skin. 


Location: butt. Eyepiece: none. 

Thickness of section: 20 wu. Objective: 32-mm. 

Stains: Van Heurck’s logwood, Wratten filter: F-red. 
Daub’s bismarck brown. Magnification: 40 diameters. 


49 


50 THE CHEMISTRY OF LEATHER MANUFAC 


skin is noticeable. This fact is doubly interesting because the structure 
of this layer is of much greater importance for calf skin than for 
cow hide; calf skins are generally used to make dressing and other 
leathers where fineness of appearance of the grain surface is highly 
valued, while cow hides more often are used for sole, belting, and harness 
leathers. 

Another point to be noted in comparing Figs. 11 and 18 is that 
the sections were cut from exactly corresponding parts of the skins 
of the two animals. The importance of this point will be made clear 
from a study of Figs. 20 to 27. It is well known that a tanned skin 
is not uniform in structure throughout its entire area. The ‘butt is 
usually much thicker and has greater solidity than any other part. The ~ 
shanks are firm, but thin, while the flanks are thick, but spongy. In 

order to show how the 
structure of the skin varies 
in different regions, 8 strips 
were cut from the locations 
indicated in the diagram 
shown in Fig. 19. The 
skin was the same as that 
whose vertical section is 
shown in Fig. 18. Vertical 
sections of these 8 strips 
are shown in Figs. 20 to 27. 
In comparing the sections, 
it will be noted that the 
thickness of the thermostat 
layer is uniform through- 
out the skin, but that both 
~ the thickness and texture of 


Fic. 19—Diagram of Calf Skin showing loca- the reticular layer vary 
tions of sections whose photomicrographs widely. 


are shown in Figs. 20 to 27. The reticular layer is 
- ee shank ; ee ae shank ; nearly 3 times as thick in the 
seuencuidert PAs ete butt as in the hind shank. In 
7: butt; 8: tail. the shoulder, the reticular 


layer is thinner than that 
of the butt and its fibers are somewhat finer. In the belly, the collagen 
fibers run nearly parallel to the grain surface and offer little resistance 
to any tendency to pull them apart in a vertical direction, whereas 
many of the fibers in the butt run nearly vertically, with some running 
in almost any direction, making this region very resistant to distortion. 
The grain surface appears less serrated on the butt than elsewhere. 
In fact, most of the differences observable in the various parts of 
finished leather may be attributed to initial differences in structure 
of the living skin. 
In studying Fig. 18, use may be made of practically the entire 
description of cow hide given above. The bottom fifth of the picture 
shows the adipose tissue, consisting of rows of fat cells held together 


mimeo LOG SLOP eS KIN 51 


by strands of connective tissues. The thick band forming the lower 
boundary of the derma is closely interwoven with elastin fibers, but 
between this region and the thermostat layer, as in the cow hide, there 
are very few elastin fibers. 

A better view of the fibers of the reticular layer may be had by 
referring to Fig. 8, which shows some of the fibers appearing at the 
left hand side of Fig. 18, but at a much higher magnification. 


Sheep Skin. 


Fig. 28 shows a vertical section of the skin of a healthy sheep, 

fixed in Erlicki’s fluid immediately after the death of the animal. Its 
' structure is very different from that of the calf skin, both in the 
thermostat and reticular layers. A comparison of Figs. 18 and 28 
indicates very plainly why sheep skin cannot be substituted for calf 
skin, where firmness and substance are desired. The collagen, or 
leather-forming, fibers of the sheep are extremely thin and not closely 
interwoven and tend to run parallel to the skin surface, which in itself 
makes for looseness of texture. Moreover, in the thermostat layer 
there are numerous sweat glands and fat cells, which leave empty spaces 
in the finished leather and make it very spongy 

The proportion of fat cells to collagen fibers in sheep skins varies 
considerably according to the feeding of the animal, and there is often 
to be: found an almost continuous layer of fat cells separating the two 
main layers of the skin. In such cases, it 1s desirable to separate the 
skin into its two layers before tanning and to tan each separately 
rather than to try to keep them together. Usually the skins are split 
into two parts after the liming process and the thermostat layers, called 
grains, are tanned with sumac or other tanning extract to make leather © 
suitable for bookbinding, hat bands, etc., while the reticular layers are 
converted into chamois leather, for which they are particularly suitable, 
by means of a tannage with cod oil. 

The dark, curved mass, very prominent in the upper, right hand 
of the picture and the smaller masses of similar appearance are por- 
tions of hair follicles. Unlike the follicles of the calf, those of the 
sheep turn and twist in every direction. We were unable to find 
one follicle lying wholly in a single plane. The curvature of these 
follicles is responsible for the curliness in the wool of the sheep. In 
_ the cow and calf, the hair is straight because the follicles are straight. 

The twisting of the follicles makes the study of the structure of 
sheep skin more difficult than that of the calf. But the examination 
of several sections is sufficient to show that the general mechanism 
of the two skins is the same. Running from the top of the portion of 
hair follicle showing in the upper right part of the picture is a part of 
an erector pili muscle. The sebaceous glands appear to be very near 
the surface, while the sweat glands occupy much of the lower portion 
of the thermostat layer. 

Sections from this skin at different stages of the tanning processes 
are shown in Chapters 8, 9, and 13 and should be examined in con- 


if 


y 


we 


"he. " GER 
Bete 
ee, 


Fic. 20.—Fore Shank.. Fic. 21.—Hind Shank. . 
Fic. 22.—Neck. Fic. 23.—Belly. 
Vertical Sections of Calf Skin. : 
Locations: as noted. Eyepiece: none. 
Thickness of sections: 15 wu. Objective: 32-mm. 
Stains: Van MHeurck’s logwood, Wratten filter: F-red. 
Picro-indigo-carmine. Magnification: I5 diameters. 


52 


SON pgp Sys ERR 


Fic. 24.—Shoulder. Fic. 25.—Backbone. 
Fic. 26.—Butt. Fic. 27.—Tail. 
Vertical Sections of Calf Skin. 
Locations: as noted. Eyepiece: none. 
Thickness of sections: 15 wu. Objective: 32-mm. 
Stains: Van MHeurck’s logwood, Wratten filter: F-red. 
Picro-indigo-carmine. Magnification: 15 diameters. 


53 


Fig. 28.—Vertical Section of Sheep Skin. 


Location: butt. Eyepiece: none. 

Thickness of section: 20 p. Objective: 16-mm. 

Stains: Van MHeurck’s logwood, Wratten filter: C-blue. 
Daub’s bismarck brown. Magnification: 50 diameters. 


34 E 


Fig. 29.—Vertical Section of Kid Skin. 


Location: butt. Eyepiece: none. 

Thickness of section: 25 w. Objective: 16-mm. 

Stains: Van MHeurck’s logwood, Wratten filter: H-blue green. 
Picro-indigo-carmine. Magnification: 50 diameters. 


55 


56 -THE CHEMISTRY OF LEATHER MANUFACTURE 


nection with the study of the raw skin. The epidermis can be differ- 
entiated more clearly by comparing Fig. 28 with Figs. 66 and 67 of 
Chapter 8. The general arrangement of the elastin fibers is best shown 
in Fig. 83 of Chapter 9. 3 

The specimen of sheep skin shown was unusually free from the 
fat cells that tend to separate the skin into two layers. We were 
able to tan it into a reasonably firm piece of leather, A section of 
this leather is shown in Fig. 104 of Chapter 13. The leather was soft 
and somewhat spongy, but is probably a good example of the type 
of skin often substituted for kid skin in the manufacture of glove 
leather. 


Goat Skin. 


In many respects the skin of the goat may be regarded as having a 
structure intermediate between that of the calf and the sheep. The 
fibers are fuller and firmer than those of the sheep, but are hardly 
equal to those of the calf. The glands and fat cells, which are re- 
sponsible for the sponginess of sheep leather, are very much less 
abundant in goat skin, although it must be admitted that this is largely 
dependent upon the animal’s feeding. Both the goat and the sheep 
skins of the general market vary widely in quality and substance, a 
fact which warrants a considerable extension of the study of their 
structures. Calf skins, on the other hand, do not vary in quality 
nearly so widely. | 

Like the calf, the goat has straight follicles, and, consequently, 
straight hair. The surface of goat skin is very much coarser than 
that of calf skin. A glance at Fig. 9 will show that the pattern of 
the calf grain is considerably finer, even than that of the kid. Rough- 
ness of grain, however, is sometimes desirable and the grain surface of 
goat skins is often made still coarser by mechanical means, 

A vertical section of kid skin is shown in Fig. 29. This was just 
an average domestic skin in the condition in which fresh skins are 
usually received at the tannery. The epidermis is the very thin dark 
line forming the upper boundary of the skin. It dips down into the 
derma, forming a nearly straight follicle, in which the hair grows. 
The erector pili muscle is the thin fine running upward to the right 
from the base of the follicle. The opening of the sebaceous glands into 
the follicle can be seen just above the erector pili muscle. The fact 
that the collagen fibers run nearly parallel to the surface gives this 
skin, in its most solid part, a softness and looseness found only in 
the flanks of the calf skin. | 

Bounding the lower surface of the derma is a layer of striated 
muscle tissue, which permits the animal to twitch its skin. Muscles 
of this kind are often found on most of the various kinds of skins 
used for making leather. 

A typical section of chrome tanned goat skin is shown in Fig. 146 
of Chapter 14. It is interesting to compare its general structure 
with those of the calf and sheep. 


MISPLOLOGY OF SKIN 57 


Hog Skin. 


The comparatively low value of hog skin for leather manufacture 
can be appreciated by studying the section shown in Fig. 30. The 


Fig. 30.—Vertical Section of Hog Skin. 


Location: butt. Eyepiece: none. 

Thickness of section: 20 uw. Objective: 48-mm. 

Stains: Friedlander’s logwood, Wratten filter: C-blue. 
Daub’s bismarck brown. Magnification: 14 diameters. 


reticular layer is composed chiefly of fat cells, which have practically 
no value in making leather. We have here a case where the general 
use of the term reticular is apt to be misleading. ‘The fat cells extend 


58 THE CHEMISTRY OF LEATHER MANUFACTURE 


even up into the thermostat layer. The close relation of this structure 
to that of the human scalp, shown in Fig. 1, should be noted. 

The epidermis, as well_as the upper surface of the derma, is very 
rough and irregular in appearance. As in other skins, the epidermis 
dips down into the derma, forming the follicles in which the hairs, 


or rather bristles, grow. The hair bulbs are imbedded in the mass 


of fat cells which make up the reticular layer. These fat cells ex- 
tend higher up into the thermostat layer in the region of each hair 
follicle, about which the fat cells form cone-shaped masses. The 
structure of a hair bulb from the hog is shown in Fig, 5. 

The erector pili muscle belonging to the follicle shown in Fig. 30 
did not lie in the plane of the section. A portion of one of these 
muscles can be seen in Fig. 84 of Chapter 9, which, because of its 
very much higher magnification, also shows the arrangement of the 
elastin fibers of the thermostat layer. The hog has relatively much 
fewer elastin fibers than the cow, calf, or sheep. : 

The roughness of the surface of the derma is further accentuated 
by the presence of papillae, which seem to be rare in the skins of 
most of the lower animals studied. In the cow hide, papille were 
found only in the region of the legs, while in the calf, sheep, and 
goat skins, no papillae were found at all. It would be interesting 
to determine whether the abundance of papille makes the hog more 
sensitive to touch and pain than the other lower animals. The ex- 
treme roughness of the grain surface of tanned hog skin is very 
noticeable in Fig. 9. 

After the skin has been unhaired and prepared for tanning, only 
a portion of the thermostat layer remains. The follicles then are simply 
pockets lined with the grain membrane, the lower portions protruding 
out from the under side of the skin. When the tanned skin is shaved 
down on the under side to make it smooth, the bottoms of these pockets 
are cut away, leaving holes wherever there were bristles in the original 
skin. This serves further to lower the value of leather made from 
hog skin. A section of tanned hog skin is shown in Fig. 107 of 
Chapter 13. 


Horse Hide. 


The outstanding peculiarity of horse hide lies in the reticular layer. 
In the region of the butt there is a dense mass of collagen fibers in 
the reticular layer so compact as to render leather made from the 
butt naturally waterproof and nearly air tight. A section of horse 
hide taken from the butt is shown in Fig. 31. The dense mass of 
fibers, often called the glassy layer, can be seen running horizontally 
across the middle of the picture and appearing much darker than the 
remaining fibers. The portion of the hide containing the glassy layer 
is known as the shell and is used to make the leather sold under the 
name of cordovan. The rest of the hide not only does not have this 
glassy layer, but the fibers of the reticular layer are very loosely inter- 


} 


Fig. 31.—Vertical Section of Horse Hide. 


Location: butt. Eyepiece: none. 

Thickness of section: 20 wu. Objective: 32-mm. 

Stains: Van MHeurck’s logwood, Wratten filter: C-blue. 
Daub’s bismarck brown. Magnification: 25 diameters. 


59 


60 THE CHEMISTRY OF LEATHER MANUFACTURE 


woven, giving the leather made from it a spongy substance that limits 
1US “115e; 

The thermostat layer of horse hide resembles that of cow hide. 
The general arrangement of the hair follicles, the erector pili muscles, 
and the sebaceous glands can be seen in Fig. 31, but the full detail 
shown in the sections of cow hide is lacking because the specimens of 
horse hide were not fixed immediately after the death of the animal, 
as in the case of the cow hide. The section, however, represents a 
hide in probably the usual condition in which horse hides are received 
at the tannery. 

Figs. 105 and 106 of Chapter 13 show a comparison of leather made 
from the shell and that made from the portion of hide immediately 
adjoining the shell. In splitting the leathers to a nearly uniform 
thickness, the knife of the splitting machine cuts through the lower 
part of the glassy layer. ‘lhe greatest contrast between the two 
specimens is thus shown in the lower portions. 


Guinea Pig Skin. 


A section of guinea pig skin is shown in Fig. 32 as an example 
of very small skins. Such skins can be made into fairly good leather, 
but their diminutive size limits the demand for them and it is question- 
able whether such leather could be sold at a profit. A point worthy 
of note is that the thermostat layer of the guinea pig skin is of 
practically the same thickness as that of a calf skin, which is very 
much larger. As shown in the description of the different parts of 
the calf skin, when nature provides a thinner skin, she does so almost 
entirely at the expense of the reticular layer, and not of the thermostat 
layer. It is possible that a mininium thickness for any size of animal 
is required for the proper operation of this important layer. 

The corneous layer of the epidermis appears like a few strands 
of delicate threads just above the Malpighian layer, the dark line 
bounding the upper side of the derma. ‘The collagen fibers of the 
reticular layer are so fine that they appear only as thin threads even 
at a magnification of 70 diameters. The dark band crossing the bottom 
of the picture is a mass of striated muscle tissue. 


Fish Skins. 


The detailed structure of fish skins is very different from those 
of mammals. Nevertheless fish skins yield a leather comparing favor- 
ably with some of the more common types of commercial leathers. Fish 
leather is very tough, as a rule, and is suitable for many purposes 
where great strength is required. Sturgeon leather used for lacing 
heavy belts together has been known to outwear the belts. Tt je 
said that the people of New England, in the old days, made shoes 
and gloves from the skin of the cod fish. Other fish skins are sometimes 
used for making fancy leathers. 

In Pigs.°33, 345° andeahaare photomicrographs of sections of the 


Fig. 32.—Vertical Section of Guinea Pig Skin. 


Location: butt. Eyepiece: none. 

Thickness of section: 30 p. Objective: 16-mm. 

Stains: Van Heurck’s logwood, Wratten filter: F-red. 
Picro-indigo-carmine. Magnification: 70 diameters. 


61 


Fig. 33.—Vertical Section of Halibut Skin. 
Fig. 34.—Vertical Section of Cod Fish Skin. 
Fig. 35.—Vertical Section of Salmon Skin. 


Location: side. Eyepiece: none. 

Thickness of sections: 20 u. Objective: 32-mm. 

Stains: Friedlander’s logwood, Wratten filter: H-blue green. 
Picro-indigo-carmine. _ Magnification: 17 diameters, 


62 


/ 


Ve a ae ee ee 


| 
. 


see 
a 


iy, Made Sika <i 


ie ae I ee WER 


Fig. 36.—Vertical Section of Salmon Skin. 
Location: side. bea 7.5%. 
Thickness of section: 20 uw. Objective: 16-mm. 


Stains: Friedlander’s logwood, Wratten filter: H-blue green. 
Picro-indigo-carmine, Magnification: 185 diameters, 


64 THE CHEMISTRY OF LEATHER MANUFACTURE 


skins of the halibut, cod, and salmon. These skins have a thin epi- 
dermis covering them and which dips into the derma here and there 
forming follicles in which the scales grow. The scales of the fish 
correspond to the hairs of the warm blooded animals. The scales may 
be recognized by their saw-tooth edges. 

A portion of the right hand side of Fig. 35 is shown in Fig. 36 
at a very much higher magnification so as to show the detailed struc- 
ture of the derma. The upper portion of the picture is occupied by 
the lower end of a scale. We have not yet identified in fish skin the 
machinery of a thermostat layer like that common to the skins of 
mammals and, being cold blooded, they probably have none. Instead 
of interlacing bundles of collagen fibers, ribbons of collagen running 
parallel to the surface make up the major portion of the skin. These 
ribbons do not interlace, but here and there we note bands of collagen 
running vertically through the skin. This adds greatly to the strength 
of the skin and prevents the distortion made possible in a vertical 
direction where all the fibers or ribbons run horizontally. 

A section of tanned salmon skin, with the epidermal system com- 
pletely removed, is shown in Fig. 108 of Chapter 1 3. This leather i. 
purposely shown in the unfinished state because the structure is thus 
shown more clearly. In finishing such leather, either the loose, upper 
portion is rolled out smoothly and coated with a finishing material or 
it is shaved off and the under portion is treated with a suitable finish 
and embossed or plated. 


Other Skins. 


The descriptions of skin structure given above are the result of 
an investigation still in progress and far from complete. But it appears 
that what has thus far been accomplished represents a real advance 
and that it is desirable to present as much as possible of what has 
been learned to date, even though the subject is incomplete. In the 
chapters on tanning, sections of leather appear which were made from 
skins of which we have not yet made a study. These may profitably be 
consulted in connection with the study of histology. 

In Fig. 110 is shown a section of alligator leather. The structure 
of the fibers, or ribbons, in many ways resemble those of the fishes. 
A somewhat similar structure may be noted in the section of shark 
leather in Fig. 109. The uninviting hooks on the surface of the shark 
leather are hardly visible to the naked eye and give the leather a harsh 
feel. There are, of course, many kinds of sharks and it is not cus- 
tomary to leave these hooks on the leathers placed on the market. The 
fibrous structure of the horned-toad leather shown in Fig, 111 also 
resembles that of the fishes. In contrast to the smaller skins is the 
section of hippopotamus leather shown in Figs. 11 5 and 116.° Other 
interesting types of leather are those of the camel and walrus, shown 
in Figs. 112, 113, and 114. 


Chapter 3. 


Chemical Constituents of Skin. 


By far the greater portion of the solid matter of the skin con- 
sists of protein matter. The proteins forra one of the most important 
and complex groups of organic compounds and are remarkable for 
the number of general physical and chemical properties which they 
possess in common and the extreme difficulty of making quantitative 
separations of the several members of any one group. They all con- 
tain carbon, hydrogen, nitrogen, and oxygen, and many of them also 
contain sulfur and phosphorus. They are all amphoteric, combining 
with both acids and bases, and those that do not dissolve in water 
swell by absorbing water. They are more or less readily hydrolyzed 
by boiling acid or alkaline solutions or by appropriate solutions of 
enzymes. Hydrolysis proceeds in steps yielding in turn bodies of 
decreasing complexity, the proteoses, peptones, polypeptides, and finally 
simple amino acids. Amines and ammonia are often found among the 
various hydrolytic products. The following amino acids have been 
isolated and identified from the hydrolytic products of different 
proteins: ? 


Isoleucine, a-amino-B-methyl-B-ethylpropionic acid, (CH,.CH. 
fa. yea NH) COOH. 
Phenylalanine, B-phenyl-a-aminopropionic acid, C,H;.CH,. 
PB ENH,).COOH. 
7. Tyrosine, B-parahydroxyphenyl-a-aminopropiomc acid, HO. 
Serie. CH(NH;,).COOH. | 
8. Serine,  6-hydroxy-a-aminopropionic acid, CH,(OH).CH 
(NH,).COOH. 
g. Cystine, di-(B-thio-a-aminopropionic acid), HOOC.CH(NH,). 
ie>-5.CH,.CH(NH,)-COOH. 
10. Aspartic acid, aminosuccinic acid, HOOC.CH,.CH(NHz2). 
COO. 


1Cf. Chemical Constitution of the Proteins. R. H. A. Plimmer. Longmans, Green & - 
Co., London. 
65 


1. Glycine, aminoacetic acid, NH,.CH,.COOH. 

2. Alanine, a-aminopropionic acid, CH,.CH(NH,).COOH. 

3. Valine, a-aminoisovalerianic acid, (CH,;)2:CH.CH(NH,). 
COOH. 

A. Leucine, a-aminoisocaproic acid (CH,)2:CH.CH,.CH(NH,). 
PGOOH. 

vp 

6. 


66 THE CHEMISTRY OF LEATHER MANUFACTURE 


11. Glutamic acid, a-aminoglutaric acid, HOOC.CH,.CH,.CH 
(NH,).COOH. 

12. Arginine, a-amino-5-guanidinevalerianic acid, HN:(C.NHz). 
NE UCH. Cre Ghee Ch UNE Cer aise 

13. Lysine, o-e-diaminocaproic acid, NH,..CH,.CH,.CH,.CHz2. 
CH(NH,).COOH: 

14. Caseinic acid, diaminotrioxydodecanic acid, Ci,2H..N,O;. 

15. Histidine, B-iminazolyl-a-aminopropionic acid, 


CH 
Un 
N NH 


Rae! 
HC — ClCH CE CNT pisos 
16. Proline, a-pyrrolidinecarboxylic acid, 
EEG dab (CH. 


[eae 
HG c GH GOOCH: 
as 


NH 


17. Oxyproline, oxypyrrolidinecarboxylic acid, C;H,NO,. 
18. Tryptophane, B-indole-a-anunopropionic acid, 


NH 


C.CH,.CH(NH,).COOH. 


Under suitable conditions, amino acids can be made to combine 
with each other by removing the elements of water, the amino group 
of one combining with the carboxyl group of another, thus 


CH,.CH(NH,) .COOH + NH,.CH,.COOH = CH,.CH(NH,). 
CO.NH.CH,.COOH--H.O. 


A combination of two amino acids is called a dipeptide, one of three a 
tripeptide, etc. Fischer * succeeded in preparing the octadecapeptide 


NH, . CH(C,H,) . CO. [NHCH,CO], . NH . CH(C,H,) . CO. 
[NHCH,CO],.NH.CH(C,H,).CO.[NHCH.CO],.NH . 
CH,.COOH, 


which contains 15 glycine and 3 leucine residues and has a molecular 
weight of 1213. It gives the biuret test for protein, is precipitated from 
solution by tannin, and would have been classed as a protein had it 
been found in nature. Later Abderhalden and Fodor? succeeded in 

? Synthesis of Polypeptides. Emil Fischer. Pr. Chem. Soc. 23, 82; C. A. 1 (1907), 1545. 


3; 
® Synthesis of Polypeptides of High Molecular Weight from Glycocoll and 1-Leucine. 
E. Abderhalden and A. Fodor, Ber, 49 (1916), 561; C. A. 10 (ome f sexing 


CHEMICAL CONSTITUENTS OF SKIN 67 


preparing a polypeptide containing 15 glycine and 4 leucine residues 
and having a molecular weight of 1326. 

The close resemblance of the more complex polypeptides to the 
natural proteins and to their first products of decomposition, the pro- 
teoses and peptones, and the fact that all proteins yield amino acids 
upon complete hydrolysis have established the view that the general 
structure of proteins is at least similar to that of the polypeptides. 
The above list of amino acids indicates the tremendous number of pos- 
sible combinations to form proteins and of the isomeric forms that 
any individual protein may have. 

The generally accepted methods of classifying proteins are based 
upon differences in solubility, speed of hydrolysis, and precipitability 
under definite conditions. But, since a small amount of foreign matter 
may alter these properties entirely for a given protein and because of 
the difficulty of separating and purifying proteins, this system of 
classification is not wholly satisfactory, although it is, perhaps, the 
best available at the present time. The common names applied to pro- 
teins, such as keratin, albumin, etc., do not represent individual sub- 
stances, but groups of closely related proteins whose quantitative 
separation is very difficult. 

The most important classes of skin’ proteins, in the order of in- 
creasing importance to the tanner, are the mucins, albumins, globulins, 
melanins, keratins, elastins, the unnamed proteins of the grain surface, 
and the collagens. [Except in the case of fur skins, the first five classes 
are of importance only because they must be removed from the skin 
prior to tanning, without injuring the remaining protein matter. In 
general, the albumins are the only skin proteins soluble in pure water. 
The globulins are soluble in dilute salt solutions and the mucins and 
melanins in dilute alkalies. The four remaining classes, which belong 
to the general group of proteins known as albuminoids, are insoluble in 
dilute solutions of acids, bases, or salts at room temperature, but all are 
dissolved and hydrolyzed by boiling solutions of concentrated acids or 
alkalies. The keratins are dissolved by strongly alkaline solutions be- 
fore the remaining three classes are seriously attacked and the elastins 
are easily dissolved by trypsin before any injury is done to the collagen 
or grain surface. In boiling water, the collagen goes into solution as 
gelatin, leaving behind a residue of elastin and the proteins of the grain 
surface. 

The albumins and globulins are found in the blood and lymph of the 
skin and also in the fluids of the muscles and nerves. By extracting 
powdered dog skin with a 10-per cent solution of sodium chloride, 
under toluene at 37° C., Rosenthal * obtained a quantity of albumins and 
globulins which, upon coagulating, washing with water, alcohol, and 
ether, and drying, gave a weight equal to 24 per cent of the total protein 
of the skin. But a yield of only 4.2 per cent was obtained from calf 
skin. 

The albumins are soluble in pure water or in dilute solutions of 


2 * Biochemical Studies of Skin. G, J. Rosenthal. J. Am. Leather Chem, Assoc. 11 (1916), 
403. 


68 THE CHEVISTRYOF LEATHER MANUFACTURE 


acids, bases, and salts, but are precipitated by the addition of con- 
centrated mineral acid or by saturating a weakly acid solution with salt. 
Their solutions coagulate upon boiling, in the presence of a small 
amount of salt. 

The globulins generally are insoluble in pure water at the neutral 
point, but dissolve in dilute neutral salt solutions, from which they can 
be precipitated by sufficient dilution or by saturating the solution with 
salt, being most readily soluble in salt solutions of moderate concen- 
tration. They dissolve freely in dilute solutions of acids and alkalies. 
Like albumins, their solutions coagulate upon heating. Fibrinogen, an 
important constituent of the blood, is usually classed as a globulin, but 
differs from serum globulin in being precipitated from solution by a 
lesser concentration of neutral salt and of coagulating at a lower temper- 
ature. It tends to clot upon exposure to air, forming the insoluble © 
protein fibrin, which action is favored by rise of temperature or agita- 
tion and is hindered by cooling or the addition of acids, alkalies, or 
concentrated salt solutions. The clotting action is supposed to be due 
to the action of an enzyme, thrombin, which is not a normal constituent 
of blood, but which is formed from the leucocytes and blood plates in 
the presence of calcium salts. . 

The mucins are conjugated proteins, of the group known as glyco- 
proteins, containing both protein and carbohydrate groups in their 
molecules. They are insoluble in pure water, but, in faintly alkaline 
solution, give mucilaginous solutions which are precipitated by the addi- 
tion of acid. It is questionable whether mucins are abundant in the 
skins of mammals. It has often been assumed that the mucins form 
the elusive “interfibrillary cementing substance” of the skin, but the 
existence of a cementing substance in the fibers, other than collagen 
itself, has not been clearly demonstrated. 

Rosenthal ® extracted calf skin, previously freed from albumins and 
globulins, with half-saturated lime water under toluene. Upon render- 
ing the extract acid with hydrochloric, protein matter was precipitated, 
which was washed with dilute acid, water, alcohol, and ether, and 
dried and weighed. The yield of protein, which he called mucoid, 
equalled about 2.7 per cent of the total protein matter of the skin. 
The yield from the solid part of the butt was 4.8 per cent against only 
1.2 per cent for the loose portions of the belly. Although mucoids 
are dissolved by dilute alkalies and precipitated by rendering the solu- 
tion acid, doubt is thrown on Rosenthal’s interpretation of his results 
by the experiments of Thompson and Atkin,® who showed that hair 
and wool are partly dissolved by lime liquors and that some of the 
matter dissolved is precipitated by rendering the solution slightly acid. 
Since the newly formed epithelial cells are very much more easily at- 
tacked than hair and wool, much of the material isolated by Rosenthal 
may actually have been derived from this source. | 

No very sharp line of distinction can be drawn between the mucins 


5 Loc. ctt. : 
* Note on the Analysis of Lime Liquors. F. C. Thompson and W. R. Atkin. J. Soc. 
Leather Trades’ Chem. 4 (1920), 15. 


PevICAE CONSTITUENTS OF SKIN 69 


and the mucoids. Hammarsten ‘ differentiates between them as follows: 
“The true mucins are characterized by the fact that their natural solu- 
tions, or solutions prepared by the aid of a trace of alkali, are mu- 
cilaginous, ropy, and give a precipitate with acetic acid which is 
insoluble in excess of acid or soluble only with great difficulty. The 
mucoids do not show these physical properties, and have other solu- 
bilities and precipitation properties.” 

The melanins are proteins of intense color, usually reddish-brown 
to black, constituting the pigment of the hair and epithelial cells. They 
are insoluble in water and dilute acids, as a rule, but dissolve more 
‘or less readily in dilute alkalies. They may be extracted with boiling 
dilute alkali and precipitated by the addition of acid. They contain 
variable amounts of iron and sulfur in combination. 

The origin of the melanins is not known with certainty, although 
it seems probable that they are derived from the blood and lymph. 
Their development is accelerated by frequent exposure to strong sun- 
light. Prolonged exposure is followed by a rush of blood to the skin 
and the production of pigment to protect the tissues against the action 
of the intense light. This shows itself in the apparent darkening of 
the color of the skin. The coloring matter of the blood, hemoglobin, 
belongs to the class of conjugated proteins known as chromoproteins 
and, like the melanins, also contains iron and sulfur. 

That the blood and lymph contain substances capable of reacting 
to produce deeply colored bodies is well appreciated by the tanners. 
Skins from which the blood and lymph have not been washed are 
liable to develop stains very difficult to remove, unless special pre- 
cautions are taken, which will be discussed in Chapter 6 in connection 
with the preservation of skin to be kept for a considerable period 
before tanning. 

The chief constituent of the epidermal system, including the epi- 
dermis, hair, and epithelial cells of the glands, is the class of proteins 
known as keratin. The general method of preparing this material for 
examination is to boil the finely divided sample containing it with 
water and then to digest the residue with an acid pepsin solution fol- 
lowed by an alkaline trypsin solution and then to wash it thoroughly 
with water, alcohol, and finally with ether. 

Keratin differs chemically from other classes of proteins in yield- 
ing a comparatively large amount of cystine, upon hydrolysis. In the 
following table are given the yields of amino acids obtained from 
keratins from different sources along with those from samples of 
elastin and collagen, or gelatin. The differences shown by keratins 
from different sources is interesting, but each sample analyzed probably 
consisted of a mixture of different keratins more or less contaminated 
by other proteins. 

Keratin prepared in the manner described above is naturally very 
resistant to the action of dilute acids and alkalies, pepsin, trypsin, and 
boiling water, but it is dissolved by strong alkalies and by water heated 


7 Physiological Chemistry. O. Hammarsten. Translation by J. A. Mandel. John Wiley 
& Sons, New York, 


70 THE CHEMISTRY OF.LEATHER MANUFACTURE 


TABLE T. 
Per Cent Amino Acid Obtained from 
Keratin from Collagen 
Horse Sheep Sheep Goose or 

Amino Acid Hair® Wool® Horn® Feathers” Elastin™ Gelatin” 
Glycine) 20. tore ee ny 0.6 0.5 2.6 25.8 o5.50F 
WARMING 4 ee eee 1.5 4.4 1.6 1.8 6.6 8.7 
Waltrie be fates corre ane 0.9 2.8 4.5 0.5 1.0 0.0 
POUNCE ince ie oe tere FT ct 15.3 8.0 27-5 7,1 
Seriitente yh ar eae 0.6 0.1 ES 0.4 ae 0.4 
Aspartic: acid: ~ 4 Maw. 0.3 2.3 2.5 I. sy, 3.4 
Glatamie acid: i252)... 3.7 i209 172 eae 08 © 5.8 
Cystine On faeces: 8.0 Ae 7.5 a re tng 
Phenylalanine “2... 0.0 ne 1.9 0.0 3.9 1.4 
Tyrosine ne pn i ae 3.2 2.9 3.6 3.6 OCF: 0.01 
Fooline’ 3. (aes e's 3.4 4.4 37a 3.5 L.7 9.5 
Osyproline! oneness ae < oat ae : 14.1 
Plistidigie tere: $5. oe: ge 0.6 car es a ass 0.9 
ENV ISIIIG 75 oe oy nee, oa 4.5 ee 20 i 0.3 8.2. 
LYSING eae Pee oe I.1 a3 0.2 HM Pee 5.9 


to 150° C. under pressure. The method of preparation may be criticized 
on the ground that it does not include young keratin. On the other 
hand, it may be contended that the proteins of newly formed epithelial 
cells are not keratins at first, but are later converted into keratins. 
However, the changes in properties with age are so gradual as to make 
it almost impossible to draw any sharp line of demarcation. This 
is a good example of the difficulty of trying to classify proteins strictly 
according to properties. The cells of the Malpighian layer of the 
epidermis are readily attacked by trypsin and by solutions of ammonia, 
but become very much more resistant as they are pushed upward into 
the corneous layer. 

In the stratum granulosum of the epidermis, the protoplasm of 
the epithelial cells has dried up and appears like granules inside of 
the cells. Walker ** regards these granules as consisting of two sub- 
stances, keratohyalin and eleidin, presumably stages in the transforma- 
tion of the protoplasm into the wax and fatty material with which 
the cells of the corneous layer of the epidermis are loaded. 

The yellow, elastic fibers interlacing the outer layers of the derma 
and enveloping the nerves and blood vessels are made up of a class 
of proteins called elastin. The tendons of the body have been the chief 
source of elastin used for study, in particular the ligamentum nuche, 
the tendon at the back of the head of the ox. F. L. Seymour-Jones ™ 
found that a piece of ligamentum nuche of about 1 square centimeter 
cross section gave on a testing machine an extension of 150 per cent 
before breaking, the strain being too small to measure; less than 5 lbs. 

8 Abderhalden and Wells. 2. physiol. Chem. 46 (1905), 31. ; 

* Abderhalden and Voitinovici. Jbid., 52 (1907), 348. 

30 Abderhalden and Le Count. IJbid., 48 (1905), 40. 

1 Abderhalden. Lehrbuch der physiol. Chem. (1909). 

2H. D. Dakin. J, Biol, Chem. 44 (1920), 524. 

13 Dermatology. N. Walker. Wm. Wood & Co., New York. 


* Chemical Constituents of Skin. F. L. Seymour-Jones. J. Ind. Eng. Chem. 14 (1922), 
130, 


Pee VICAL CONSTITUENTS. OF SKIN 7t 


He also found that the tendon was slowly digested by lime water, 
although the action may have been due to bacteria. 

Elastin may be prepared for study by extracting this tendon with 
dilute sodium chloride solution, washing and then boiling it with water, 
then with a 1-per cent solution of potassium hydroxide, again with 
water, and then with acetic acid. The residue is then treated with 
cold 5-per cent solution of hydrochloric acid for 24 hours, thoroughly 
washed with water, boiled again with water, and then washed with al- 
cohol and ether and dried. It then has a yellowish-white appearance. 
It is not dissolved by boiling water nor by acids and alkalies in the 
cold, but is easily dissolved by concentrated mineral acids upon heating. 
The yields of the different amino acids from a sample of elastin are 
given in Table I. 

It is, of course, not safe to assume that elastin from skin has 
exactly the same properties as that from other parts of the body, but 
the difficulty of isolating some of the skin proteins for study has 
made it desirable to investigate proteins of the same general classes 
from parts of the body where they are more easily available, if only 
to get a suggestion of the properties of the skin proteins. Actually 
we do find that the elastin of skin behaves much like that from the 
ligamentum nuche, being resistant to boiling water and to cold solu- 
tions of acids and alkalies. In glue manufacture, much of the elastin 
remains in the scutch or residue left after boiling the skin in water. 
By examining sections of skin under the microscope, after special 
treatments, we have found that the elastin fibers are not appreciably 
attacked by dilute solutions of acids and alkalies or by tannery lime 
liquors, but are easily dissolved by neutral trypsin solutions. ‘These 
fibers apparently act so as to resist an increase in area of the grain 
surface of the skin. 

The proteins of the grain surface are remarkably resistant to 
most of the ordinary chemical reagents. The thin fibers of this surface 
are not dissolved by solutions of caustic alkalies sufficiently strong 
to destroy the collagen fibers, epidermis and hair. In boiling water, 
they evidently undergo some change in composition, but remain un- 
dissolved in the form of a thin sheet while the collagen passes into 
solution as gelatin. They are apparently unaffected by trypsin solu- 
tions strong enough to dissolve all of the elastin fibers beneath them. 
But in contact with water having a pH value of about 6, they are easily 
attacked and liquefied by putrefactive bacteria, although this action 
can be checked by the addition of a sufficient amount of acid, alkali, 
or salt. 

These fibers represent only a very small proportion of the skin 
by weight, but they are of great importance because they form the 
grain surface of finished leather, giving it its characteristic appearance. 
Their position in the grain surface is shown in Fig. 150 of Chapter 106. 
In tanning and dyeing, they take a color different from that assumed 
by the collagen fibers, which is noticeable when leather is cut. Any 
damage to the grain surface reduces the selling value of the leather 
materially. 


‘9m. THE CHEMISTRYCOE eral Er MANUFACTURE 


Collagen is the most abundant protein of the skin and the one 
of greatest importance to the tanner, since it is the basis of leather. 
It constitutes the bulk of the substance of the white fibers of the 
connective tissues of the derma. 

Collagen can be prepared for study from fresh skin by removing 
the other constituents. The adipose tissue is carefully cut away and the 
skin thoroughly washed. It is then extracted with several changes 
of 10-per cent sodium chloride solution, in a closed jar set in a 
tumbling machine, or agitator, in order to remove the soluble protein 
matter. It is then put back into the same jar with a one-tenth-per cent 
solution of sodium sulfide containing lime well in excess of saturation 
and tumbled occasionally for several days, or until the hair is quite 
loose. 

The hair and epidermal matters are then removed by scraping the 
grain surface with a knife blade. The entire grain surface is then 
cut away, preferably on a splitting machine. The skin is then washed 
thoroughly to remove most of the lime and is then digested for 5 
hours at 40° C. with a solution containing 1 gram of U.S. P. pancreatin, 
2.8 grams of monosodium phosphate, and 18 cubic centimeters of molar 
sodium hydroxide per liter. This removes all of the elastin fibers. 
The skin is then cut into small pieces and put into a jar of water 
equipped with a stirring device. Hydrochloric acid is added at such 
rate as to maintain the solution just faintly acid to methyl orange. 
When no more acid is required, the pieces are left to wash in running 
tap water over night. Next day they are soaked in several changes of 
alcohol to remove the water and then in xylene, after which they are 
exposed to air until the xylene has evaporated. They are then ground 
in a mill to a fibrous powder. Collagen thus prepared is known as 
hide powder. 

Upon heating with water to 70° C., collagen slowly passes into ~ 
solution as gelatin. But just what relation gelatin bears to its parent 
substance collagen is not known with certainty. Hofmeister *® sug- 
gested that collagen is an anhydride of gelatin and that the change from 
one to the other is reversible, collagen being regenerated by drying 
gelatin at 130°C. This heating changes the properties of gelatin so 
that it swells in water to a lesser extent than before and passes into 
solution with greater difficulty. In commenting upon Hofmeister’s 
work, Alexander *® says “It is extremely doubtful if collagen is re- 
generated under these conditions, the more probable explanation being 
that, upon driving off the water, the constituent particles of the gelatin 
approach so close as to form an irreversible gel, thus rendering it 
insoluble.” 

C. R. Smith 17 found that gelatin dried at 100° C. and then heated 
to 128° loses 1.25 per cent in weight. It then swells very slowly 
and dissolves in water at 35° to 40°, with nearly complete restoration 
of its jellying power. He concedes that gelatin dried at 128° may 

16 Z. physiol. Chem. 2 (1878), 299. 

16 Allen’s Commercial Organic Analysis. Vol. 8 (1913), p. 586 


17 Mutarotation of Gelatin and Its Significance in Gelation. C. R. Smith. J. Am. Chem. 
Soc. 41 (1919), 135. 


CHEMICAL CONSTITUENTS OF SKIN 73 


be converted into collagen, but that collagen itself may represent a 
form of gelatin which is difficult to disperse. Emmett and Giles,’ 
on the other hand, suggest that the conversion of collagen into gelatin 
involves an intramolecular rearrangement. 

Plimmer ¥° says “those proteins which are resistant to the action 
of trypsin until they have been acted upon by pepsin will have all 
their units contained in the anhydride ring.” Gelatin is easily hydro- 
lyzed by either pepsin or trypsin, while it has been generally believed 
that collagen is hydrolyzed by pepsin, but not by trypsin. This led 
the author 2° to suggest that Plimmer’s statement corroborated Hof- 
meister’s view of the anhydride structure of collagen. But Thomas 
and Seymour-Jones ** have recently 
demonstrated that collagen is at- 
tacked by trypsin under the right 
conditions. The erroneous view that 
collagen is resistant to tryptic diges- 
tion unless previously swollen with 
acid or alkali dates back to a series 
of studies by Kuthne,?* Ewald and 
Kuhne,?? and Ewald,?* which were 
based only upon qualitative observa- 
tions. 

Thomas and Seymour-Jones 
found that trypsin acts most rapidly 
upon collagen at a pH value of 5.9 
and that the action is not appreciably 
accelerated by soaking the protein 
previously in solutions of higher 
‘or lower pH values such that the 
protein is not actually hydrolyzed 
by the acid or alkali. In studying _—- 
the effects of time and concentra- ; en At : 


tion of enzyme upon the digestion Fic. 37—Rate of digestion of hide 


of hide powder by trypsin, they powder by trypsin as a function of 
adopted the following precedure. time. 


In each experiment 0.5 gram of 

hide powder was placed in a centrifuge tube having a capacity of Io 
cubic centimeters and a conical bottom graduated in units of 0.1 cubic 
centimeter. In order to bring the hide powder to the optimum pH value, 
they covered it with 5 cubic centimeters of a phosphate buffer solution 
having a pH value of 5.9 and a few drops of toluene to check bacterial 
action. The tube was shaken for 3 hours, then centrifuged for 20 min- 
utes at 1000 times gravity, and the volume of hide powder read from the 


1% J, Biol. Chem. 3 (1907), 33. 

39 Loc. ett. , 
( ng lone des of Leather Chemistry. J. A. Wilson. J. Am. Leather Chem, Assoc. 12 
1917), 108. 

21 Hydrolysis of Collagen by Trypsin. A. W. Thomas and F. L. Seymour-Jones, ih ANAS 
Chem. Soc. (1923); Dissertation, F. L. Seymour-Jones, Columbia University, 1923. 

22.W., Kihne. Verhande. Naturhist. Med. Ver., Heidelberg, 1 (1887), 198. 

23 A Ewald and W. Kihne. Jbid., 1 (1887), 451. 

24 A. Ewald. Z. Biol, 26 (1890), 1. : 


Fraction of Hide Powder Digested 


74 THE CHEMISTRY OF LEATHER MANUFACTURE 


graduations in the tube. The supernatant liquor was then run away and 
replaced by 5 cubic centimeters of trypsin solution having a pH value of 
5.9 or by the buffer solution where a blank was being run. Toluene was 
added in every case as a safeguard. The solution was shaken in a ther- 
mostat at 40° C. for a stated length of time and then centrifuged and the 
volume of hide powder again read, the loss in volume being taken as a 

measure of the amount of hide powder dissolved. | 
The rate of digestion of hide powder by a 0.5-per cent trypsin 
solution is shown in Fig. 37 as a function of the time. With a solu- 
tion so concentrated in enzyme, hydrolysis takes place extremely 
rapidly. It is interesting to note - 


6 Bipe hide oonabes also the steady hydrolysis in the 
se iat 1 blank (without enzyme) at 40° C. 
time = 30 mine, In Fig. 38 are shown the rates 


of digestion of fine and coarse hide 
powders as functions of the concen- 
tration of enzyme. The fine pow- 
der consisted of the portion passing 
through a sieve of 34 meshes to the: 
inch and the coarse powder of the 
portion retained by the sieve. A 
much longer time is required to 
hydrolyze the coarse powder, as was 
expected. In Chapter 8 it will be 
shown that a concentrated solution — 
of trypsin produces marked hydro- 
lysis of calf skin only after acting 
for nearly 40 hours. Here the time 
required for diffusion of the en-’ 
100 200 300 400 500 600 zyme into the skin and complica- 
miviigrens of Trypsin per Liter tions due to the presence of preteia 

Fig. 38.—Rates of digestion of fine matter other than collagen play a 
and coarse hide powders as func- part. In the method described above 
tions of the concentration of tryp- for preparing collagen for study, the 
a action of the enzyme does not result 
in any very serious loss of collagen, but all of the elastin is digested. 
Collagen is hydrolyzed by concentrated solutions of acids and alkalies 

in the cold, if sufficient time is allowed. Upon heating the solutions, 
the hydrolysis proceeds rapidly. In a study of the hydrolysis of 
gelatin by acids, alkalies, pepsin, and trypsin, Northrop *® found that 
the course of the early stages of hydrolysis is similar with alkali, 
trypsin, and pepsin, but quite different with acid. He made a com- 
parison of the relative velocities of hydrolysis of the various peptide 
linkings and observed the following important facts. Those linkages 
which are hydrolyzed by pepsin are also hydrolyzed by trypsin; but 
trypsin hydrolyzes linkages which are not attacked by pepsin. Of 
the linkages hydrolyzed by both enzymes, those most rapidly hydrolyzed 


Fraction of Hide Powder Digested 


** Comparative Hydrolysis of Gelatin by Pepsin, Trypsin, Acid, and Alkali. J. H. 
Northrop. J. General Physiol. 4 (1921), 57. ; 


CHEMICAL CONSTITUENTS OF SKIN 75 


by pepsin are only slowly attacked by trypsin. Those linkages which are 
most rapidly split by pepsin or trypsin are among the more resistant 
to acid hydrolysis and least resistant to hydrolysis by alkali. 

The chemistry of collagen and gelatin forms so large a portion of 
the chemistry of leather manufacture that further treatment must be 
reserved for the appropriate chapters. 

The skin contains a number of non-protein substances in the blood, 
lymph, and gland secretions. The blood and lymph contain sugars, 
salts, particularly the phosphates, carbonates, sulfates, and chlorides of 
sodium and potassium, and fatty matters, including cholesterols and the 
lecithins, which are phosphorous compounds of fats often existing in 
loose combination with proteins. Sodium chloride is the chief con- 
stituent of perspiration, which also contains sulfates, phosphates, and 
urea, and sometimes sebum. Sebum, the secretion of the sebaceous 
glands, consists of cholesterols, complex oleins, higher alcohols, and 
soaps, and is usually found contaminated with epithelial cells, probably 
those of the sebaceous glands furnishing the sebum. 


Chapter 4. 


Ionization of Acids and Bases Commonly Used 
in the Tannery. | 


Of vital importance in the use of tannery liquors is the-control of 
hydrogen-ion and hydroxide-ion concentrations. Irregular variations 
in these concentrations are almost certain to result in corresponding 
irregularities in the properties of the leather produced. By juggling 
the methods of operation until a nearly uniform product was obtained 
and then rigidly adhering to a developed process, tanners long ago 
perfected means for keeping hydrogen-ion concentrations reasonably 
well under control, although without any appreciation as to why cer- 
tain steps had to be followed. If liquors suddenly became infected 
with acid-producing ferments, or got beyond the control of the operator 
from other causes, the result was apt to be disastrous unless the tanner 
had learned from similar experiences how to correct the trouble. 

Many of the pioneers who attempted to introduce chemical methods 
to the industry were handicapped by their inability to compare the 
activities of acids or bases of different strengths. Too much reliance 
upon the total concentration of acid, with little or no appreciation 
of its degree of ionization, has often proved very misleading. It is 
still not uncommon to find expensive acids being used where cheaper 
ones would serve the purpose as well or better. Even where an operator 
had come to appreciate that the determining factor was the hydrogen- 
ion concentration rather than the total titrable acidity, he was often 
without the means for determining hydrogen-ion concentrations and 
there were no easily available figures showing the degrees of ionization 
of the commoner acids and bases at different concentrations. In order 
to remedy this situation, Thomas * computed and compiled from the 
literature a series of tables showing the degrees of ionization of a 
number of acids and bases commonly used in the tannery; a range from 
0.001 to 2 molar is covered. These tables are incorporated in this 
chapter because it is believed they will make certain portions of the 
book more readily comprehensible to a greater number of readers and 
will prove of great value for reference in experimental work on leather 
manufacture. 

In making the calculations, Thomas used two modes of procedure. 
For the weak acids the concentrations of hydrogen ion have been cal- 


? Tabulation of Hydrogen and Hydroxyl Ion Concentrations of Some Acids and Bases. 
A. W. Thomas. J. Am. Leather Chem. Assoc. 15 (1920), 133. 


76 


Mevi2 alION? OF ACIDS AND BASES , 77 


culated from the ionization constants (determined by conductivity 
measurements) by means of Ostwald’s dilution law, 
a4 
V(I—a) 
where K is the ionization constant, V the volume in which I gram 


molecular weight is dissolved, and a the degree of ionization. By 
rearrangement of the equation, we get 


—KV + VK?V? 4 4KV 
Zz 


K= 


i ewe 


But, since the value of K*V? is negligible compared to KV, it can be 
dropped for the purpose of making the calculations. The following 
expression, therefore, was used: 


Per cent ionization = 100\/ KV — 5oKV. 


For the strong acids, the experimentally determined values for 1ooa 
at various concentrations were found in the literature. These were 
plotted against values for logV and a smooth curve was drawn through 
the points. The desired values were then read from the curve. The 
hydroxide-ion concentrations of bases were obtained similarly. 

The figures in the tables may be in error as much as 5 per cent, 
especially in the cases of the strong acids and bases, but they are 
the best obtainable at this time. They were obtained from conductivity 
data and not from measurements by the hydrogen electrode. 


Acids. 


Acetic Acid.—Values calculated from the experimentally deter- 
mined figures of Kendall.? 

Boric Acid.—Calculated from K = 6.6 X 10719 at 25° C. by Lun- 
den.* 0.8 molar is saturated solution and since this acid is exceedingly 
weak, only the concentrations at 0.8, 0.1, 0.01, and 0.co1 molar are 
given in the table. 

Butyric Acid.—For concentrations 2 to 0.1 molar, calculated from 
Poet tO «6at 25° by Ostwald* From 0.1 to 0.001 molar 
calculated from Ostwald’s experimental values. | 

Carbonic Acid.—This acid is very weak and its concentration in 
solution depends upon the pressure of carbon dioxide on the surface 
of the solution. For this reason no special table was prepared and 
only two significant concentrations are given here, taken from Kendall.® 
At 25° the solubility of carbon dioxide in water at 1 atmosphere of 
pressure of carbon dioxide is 0.0337 mole per liter. The carbonic 
acid in this solution is 0.33 per cent ionized and hence its concentra- 


2 Medd. Vetenskapsakad. Nobelinst, Band 2, No. 38 (1913), 1-27. 
37. chim. phys. 5 (1907), 574. 

*Z. physitk. Chem. 3 (1889), 170. 

5J, Am. Chem. Soc. 38 (1916), 1481. 


“8 THE CHEMISTRY OF LEATHER MANUFACTURE 


tion of hydrogen ion is 0.coo11 mole per liter, representing a pH value 
of 3.96. Under ordinary conditions, the partial pressure of carbon 
dioxide in the air is 0.000353 atmosphere, at which pressure carbon 
dioxide is soluble to the extent of 0.0oooo11g mole per liter, yielding a 
hydrogen-ion concentration of 0.000002 mole per liter or a pH value 
of 5.70. 

Citric Acid.—For 2 to 0.4 molar, the values of Kendall, Booge and 
Andrews ® are given. From 0.4 to 0.1 molar, the values are ex- 
trapolated. From 0.01 to 0.001 molar, the concentrations are calculated 
from the measurements of Walden.’ 

Formic Acid.—From 2 to 0.1 molar, the values are calculated from 
KS A ee ae evel ue Ostwald. From 0.1 to 0.001 
molar, they are calculated from Ostwald’s experimental determinations. 

Gallic Acid.—From I to 0.03 molar, values are calculated from 
K = 4.0 X Io, as given by Ostwald. From 0.03 to 0.001 molar, 
values are calculated from Ostwald’s experimental values. 

Hydrochloric Acid.—The figures for 2 to 0.5 molar are from 
Jones. Those for 0.5 to o.oor molar are calculated from Kohl- 
rausch’s ?° experimentally determined values. 

Lactic Acid.—The figures for 2 to 0.1 molar are based upon the 
figures of Kendall, Booge and Andrews; ® those for 0.1 to 0.001 molar 
are calculated from the experimental values of Ostwald.® 

Nitric Acid.—The 2 to 1 molar values are taken from Jones ; ° those 
for 0.5 to 0.001 molar are calculated from Kohlrausch’s ?° data. 

Oxalic Acid.—The only data available are those of Ostwald,® cover- 
ing the range only from 0.03 to 0.004 molar. This acid is too highly 
ionized to permit calculations by the dilution law. 

Phosphoric Acid.—Figures for 2 to 0.1 molar are calculated from 
the data of Kendall, Booge and Andrews; ® those from 0.1 to 0.001 
molar from the experimental data of Noyes and Eastman.™ 

Salicylic Acid.—Values are based upon the experimental data of 
Kendall.? 0.0167 molar represents the limit of solubility. 

Sulfuric Acid.—The figures for 2 to 1 molar are from Jones; ® 
those for 0.5 to 0.001 molar from the experimental data of Kohlrausch.” 

Tartaric Acid.—From 2 to 0.04 molar, the figures are calculated 
from the data of Kendall, Booge and Andrews;® from 0.04 to 0.001 
molar, they are calculated from Ostwald’s ® experimental data. 


Bases. 


Ammonium Hydroxide.—tThe figures for this weak base are calcu- 
lated, by means of the dilution law, from K = 1.8 X 10° at 25° C., as 
given by Noyes, Kato and Sosman.’. 

Barium Hydroxide—The only available data for this base are 


®°J, Am. Chem. Soc. 39 (1917), oe 

7Z. physik. Chem. 10 (1892), 5 

&Z. phystk. Chem. 3 (1889), Bh 

® Carnegie Inst. Publ., No. 60 (1907), 

10 Morgan’s Elements ‘of Physical fue hee 4th edition (1908), 519, 
11 Carnegte Inst. Publ., No. 63 (1907), 268. 

22. physik, Chem. 73 CrgTO), V1 


IONIZATION OF ACIDS AND BASES 79 


those of Noyes and Eastman," which range from 0.001 to 0.05 molar, 
upon which the calculations in the table are based. 

Calcium Hydroxide.—No series of experimental data for this base 
could be found, but it is so similar to barium hydroxide that prob- 
ably no great error would arise from the use of the barium hydroxide 
figures. 

Potassium Hydroxide.—The 2 molar value is from Jones. Values 
for I to 0.4 molar and from 0.03 to 0.001 molar are calculated from 
Kohlrausch’s '° data; those between 0.4 and 0.03 molar are obtained 
by extrapolation. 

Sodium Hydroxide-—The 2 molar figure is from Jones;° the 
others are from Kohlrausch’s !° data. 


Order of Strengths. 


Listing the acids in order of increasing strength, or hydrogen-ion 
activities, we have 


Boric 
Carbonic 
Butyric 
Acetic 
Gallic 
Lactic 
Formic 
Citric 
attaric 
Salicylic 
Phosphoric 
Oxalic 
Sulfuric 
Nitric, Hydrochloric 


Boric is the weakest acid in the list and hydrochloric and nitric are the 
strongest. 
The bases in order of decreasing hydroxide-ion activity are 


Potassium hydroxide 

Sodium hydroxide 

sarium hydroxide, Calcium hydroxide 
Ammonium hydroxide 


Temperature. 


__ All of the ‘values given in Tables II to X are for a temperature 
of 25°C. The temperature coefficient of ionization is small enough to 
be neglected for most practical purposes. The figures may, therefore, 
be considered valid for the range of temperature met with in the 
tannery. 


80 THE CHEMISTRY OF LEATHER MANUFACTURE 


pH Values. 


The term pH value is now widely used to indicate the value of a, 
with change of sign, in the expression [H"] = 10+ moles per liter. 
The use of this term has proved confusing to some because an increas- 
ing pH value indicates a decreasing hydrogen-ion concentration. But 
the pH scale has proved of great value for the operator with no knowl- 
edge of chemistry. He accepts it as a standard scale of acidity and 
alkalinity, as he does a thermometer for temperature, without caring 
about its mechanism. He learns, for example, that a given liquor 
works best at a pH value of 5.5. When the analyst reports to him 
a value for this liquor of 6.5, he immediately appreciates that the addi- 
tion of acid is necessary to bring the liquor back to 5.5. The routine 
worker adopts the pH scale almost as easily as any other system 


TABLEAH, 
Hydrochloric Acid " Nitric Acid 
Moles 
of acid Per cent Moles Ht Percent Moles H* 
per liter ionized perliter pHvalue ionized per liter pH value 
O.00 Pac dt ee 100.0 0.0010 3.00 100.0 0.0010 3.00 
OD02 Beta ee eee 100.0 0.0020 2.70 99.5 0.0020 2.70 
O004 2 kn fee 100.0 0.0030 252 99.5 0.0030 2.52 
OO0A Tins oa 100.0 0.0040 2.40 ° 99.4 0.0040 2.40 
DOOGL i tee ee 100.0 0.0050 2.30 99.4 0.0050 2.30 - 
C.00020 sot esha. 100.0 0.0060 3.22 99.4 0.0060 2.22 
OOTP Eas Shtrase 100.0 0.0070 215 99.3 0.0070 2.15 
P0080 15s as eee 100.0 0.0080 2.10 99.3 0.0079 2.10 
G.000 726 ween cae 99.9 0.0090 2.05 99.3 0.0089 2.05 
MOUs cee panes 99.8 0.010 2.00 99.3 0.010 2.00 
O02) ccs Bt 98.8 0.020 1.70 99.3 0.020 1.70 
O03. .6 0 eee 98.0 0.029 1.54 99.2 0.030 1.52 
GA ae ern 97.6 0.039 1.41 08.7 0.039 1.41 
O08 aie ete oan 96.8 0.048 142 08.3 0.049 1.31 
AR 8 «Pompe ens ioe OR 06.4 0.058 1.24 97.6 0.059 1.23 
OTE er fete 95.8 0.067 17 97.3. + 6.068 1.17 
CV: wet htae c ee ee 95.6 0.076 Hi2 96.8 0.077 I.II 
COON eee 95.2 0.086 1.07 06.3 0.087 1.06 
re SOR EE Riba 5 Cave 94.8 0.095 1.02 96.0 0.096 1.02 
FO.Si 28 ko ae 92.0 0.184 0.74 92.9 0.185 rae ee 
ate aN ieies ieee 90.1 0.270 0.57 90.7 0.272) 2 aes 
G.dsdts oe anes 88.7 0.355 0.45 89.4 0.358 0.45 
OR eek ee ee 87.5 0.438 0.36 87.9 0.439 0.36 
OD eins cies 86.5 0.519 0.28 aie gee Soa 
OF eee 84.7 0.593 0.23 
OST VEet eee Loe 83.3 0.666 0.18 
Oud, ene os 81.5 0.734 0.13 
EO oo cae 709.6 0.796 0.10 84.8 0.848 0.07 


D0 Norte oe ee 69.3 1.386 0.54 73.0 1.478 —0.17 


IONIZATION OF ACIDS AND BASES SI 


of measurement. He soon learns that pH = 7 represents a neutral 
solution, that values increasing from 7 indicate an increasing alkalinity 
and values decreasing from 7 an increasing acidity. 

The investigator in leather chemistry finds it logical to plot variables 
against —log[H"] rather than against actual hydrogen-ion concentra- 
tions because of the enormous range covered. For him the adoption 
of the pH scale has the advantage of eliminating the use of negative 
values and making his system of record conform to one more desirable 
for making plant reports, where the use of logarithms, negative values, 
and conceptions of ionization are often apt to lead to hopeless confusion. 

The pH values corresponding to the various hydrogen-ion concen- 
trations have been added to Thomas’ tables in order to increase their 
usefulness. 


CASLE LUT. 
Sulfuric Acid * Phosphoric Acid 
Moles 
of acid Per cent Moles H* Percent Moles Ht 
per liter ionized perliter pHvalue ionized  perliter pH value 
PEQOT ONE Ge 6 a wu iste 07.7 0.0020 2.70 89.0 0.0009 3.05 
RIO os es erate were DAF 0.0038 2.42 83.0 0.0017 Pte 
SNA eS ores oe o's 90.5 0.0054 2.27 ee 0.0023 2.64 
TE a eae 88.0 | 0.0070 2.15 73.5 0.0029 2.54 
ORT tases «oes s 85.90 0.0086 2.07 70.0 0.0035 2.46 
Dis aes 84.2 0.0101 2.00 67.5 0.0041 2.39 
ia <j re 82.7 0.0116 1.04 65.0 0.0046 2.34 
lS ie ar 81.8 0.0131 1.88 63.0 0.0050 2.30 
Marin vin ie 80.5 0.0145 1.84 60.5 0.0054 2:27 
Dees wai le am 709.6 0.016 1.80 59.0 0.006 2.23 
Tyo) 55 aa a aa 73.1 0.029 1.54 47.5 0.010 2.00 
Oe Ss ere en's 69.4 0.042 1.38 42.0 0.013 1.89 
5 2.0] See eR ara 66.8 0.053 1.28 38.0 ~~ 0.015 1.82 
ne eeedes shies es 64.8 0.005 1.19 35.0 0.018 1.74 
TICE oa eae 63.5 0.076 1.12 33.0 0.020 1.70 
Ct a ki. G24~. 0.087 1.06 31.0 0.022  -1.66 
Se Sree gis, ve 61.7 0.099 1.00 ,30.0 0.024 1.62 
OO ence nd, aie tl5 -s 61.1 0.110 0.96 28.5 0.026 1.58 
Te es 2 ee 60.7 0.121 0.92 275 0.028 1.55 
i bie Seyi eee 57.6 0.230 0.64 22.8 0.046 1.34 
Ee a a 56.0 0.336 0.47 20.7 0.062 1.21 
Oo; Saas ee 54.7 0.438 0.36 19.8 0.079 1.10 
CL Sie tees ic kee.» 54.6 - 0.536 0.27 19.0 0.095 1.02 
TT tl Oe 52.9 0.635 0.20 18.8 0.113 0.95 
EU ReMi Sicbn Fo: 52.0 0.728 0.14 18.0 0.126 0.90 
OS er ork ess 51.4 0.822 0.09 17.9 0.143. > 0.84 
a nl a ee ne 50.9 0.916 0.04 17.7 0.159 0.80 
icin aoe 50.7 1.014 — 0.01 17.5 0.175 0.76 
CS EN Oe 39.9 1.596 — 0.20 16.1 0.322 0.49 


* 100 per cent ionization taken as comple te ionization into H+, Ht, and S04”. 
fj 100 per cent ionization taken as comple te ionizatidn into H+ and Il,.POy,’. 


aaa Be Ba 


Moles 
of acid 
per liter 


TABLE IV. 


Formic Acid 


Per cent Moles H* 


ionized 


per liter pH value 
0.00036 3.44 
0.00054 327 
0.00066 3.18 
0.00080 3.10 
0.00090 3.05 
0.00100 3.00 
0.00109 2.96 
0.00118 2.93 
0.00126 2.90 
0.0013 2.87 
0.0019 PAGE 
0.0024 2.62 
0.0028 2.55 
0.0032 2.49 
0.0035 2.46 
0.0038 2.42 
0.0040 2.40 
0.0042 2.38 
0.0045 Gak 
0.0064 2.19 
0.0078 2.11 
0.0092 2.04 
0.0105 1.98 
0.0114 1.94 
0.0126 1.90 
* 0.0136 1.87 
0.0144 1.84 
0.0150 1.82 
0.0206 1.69 


Per cent 
ionized 


12.8 
9.2 
ri) 
6.6 
5.9 
5.4 
5.0 
4-7 
4.4 


4.2 
3.0 
2.4 
2.1 
1.9 
i” 
1.55 
1.5 
1.4 


“4 
0.9 
0.7 
0.6 
0.57 
0.50 
0.45 
0.42 
0.40 


0.37 


0.30 


CHEMISTRY OF LEATHER MANUFACTURE 


Acetic Acid 


Moles H* 
‘per liter pH value 


0.00013 3.89 
0.00018 3.74 
0.00023 3.64 
0.00026 3.58 
0.00030 3.52 
0.00032 3.40 
0.00035 3.46 
0.00038 3.42 
0.00040 3.40 
0.00042 3.38 
0.00060 3.22 
0.00072 3.14 
0.00084 3.08 
0.00095 3.02 
0.00102 2.99 
0.00109 2.96 
0.00120 2.92 
0.00126 2.90 
0.00130 2.89 
0.00180 2.74 
0.00210 2.68 
0.00240 2.62 
0.00285 2.55 
0.00300 2.52 
0.00315 2.50 
0.00336 — 2:47 
0.00360 2.44 
0.00370 2.43 
0.00600 2.22 


. 
. 


PONIZATION OF ACIDS AND BASES 83 


TABLE. Vi; 
Gallic Acid Lactic Acid 
Moles 

of acid Per cent Moles Ht Percent Moles H* 

per liter ionized perliter pHvalue ionized =perliter pH value 
St a 18.7 0.00019 3.72 30.9 0.00031 3.51 
10 Dn 13.4 0.00027 3.57 23.0 0.00046 3.34 
I Pa 10.7 0.00032 3.49 18.7 0.00056 3.25 
Sa ¢ ae 9.3 0.00037 3.43 16.7 0.00067 3.18 
MERE Mears 5 vss 8.4 0.00042 3.38 15.1 0.00076 Baie 
“Pan Pe ee vane: 0.00046 344 13.9 0.00083 3.08 
Ue Seo 7.0 0.00049 S21 12.9 0.00090 3.05 
TCS hig as, eae ee 6.7 0.00054 ‘327, 12.2 0.000098 3.01 
PRM oe. en 6.2 0.00056 3.25 11.5 0.00104 2.98 
ER ems oe oc 5.9 0.00059 2.28 11.0 0.00110 2.06 
Tha Clr ae ae 4.1 0.00082 3.09 8.0 0.00160 2.80 
Oe een ee is 5 0 oR, 0.00099 3.00 6.6 0.00198 2.70 
SOY he oo eee 3.0 0.00120 2.92 5.8 0.00232 2.63 
GR eg once als « s 2.70 0.00135 2.87 5.2 0.00260 2.58 
CRC. 8 2.50 0.00150 2.82 4.8 0.00288 2.54 
RO eric? Sue, 3s 2.30 0.00161 2.79 4.3 0.00301 2.52 
USE ORs a 2.20 0.00176 2.75 4.1 0.00328 2.48 
TNs ee 2.05 0.00185 278 3.8 0.00342 2.47 
ERIE or eis sia 1.98 0.0020 2.70 ou. 0.00370 2.43 
es (ee eee 1.40 0.0028 2.55 ag 0.0054 2.27 
TE tee pe 1.15 0.0035 2.46 22 0.0066 2.18 
Oa eS Sis; 8s 1.00 0.0040 2.40 1.8 0.0072 2.14 
Le Sos ee 0.89 0.0045 2.35 1.6 0.0080 2.10 
a 0.80 0.0048 2.32 1.5 0.0090 2.05 
ee eS has. 0.74 0.0052 2.28 1.4. 0.0098 2.01 
Tie Ag hae 0.70 0.0056 2:25 Fr 0.0104 1.98 
Wt geo ee 0.68 0.0061 a2 1.2 0.0108 1.97 
r= 0.63 0.0063 2.20 tal 0.0110 1.96 
EAGT) i 0.8 0.0160 1.80 


84 THE CHEMISTRY OF LEATHER MANUFACTURE 


TABLE VI. 
Butyric Acip Boric Acip * 
Moles 
of acid Percent Moles H* Per cent Moles H* 
per liter ionized __ per liter pH value ionized per liter pH value 
O.00Ts ela aan II.4 0.000II 3.06 0.080 0.0000008 6.10 
O.002 7 oe oe 8.3 0.00017 3.77 rath: ae 
O.003 S358 6.8 0.00020 3.70 
O.008 ten tee 6.0 0.00024 3.62 
ODOStor ea 5.4 0.00027 3.57 
O.000 Fi. ce veo 4.9 0.00029 3.54 
CUOkst cieeree 4.55 0.00032 3.49 enees ae See 
OGOOSi0% os sorces 4.3 0.00034 3.47 ‘eit ey P tate 
ODOG Pesan ures 3.05 0.00036 3.44 Se oe eee oat 
O.0l 2o2,ce ares 3.8 0.00038 3.42 0.026 0.0000026 5.58 
0,022 in oe 27 0.00054 327 lee : pha 
OWI ee eee aoe 0.00066 3.18 
PAG wae eee 1.95 0.00078 i ee 
O05 chicos weurks re 0.00085 3.07 
Lt pe tae sees cae 1.6 0.00096 3.02 
COs des eee 1.4 0.00098 3.01 
DOG tas eee 1.35 0.00108 2.97 
00; eee ee 1.25 0.00113 2.05 
es se st atates 1.2 0.00120 2.92 0.008 0.0000080 5.10 
O37 vies sae 0.86 0.00172 2.76 ita 
iS ee ae 0.70 0.00210 2.68 
Ue: te ees Pico 0.60 0.00240 2.62 
Oe Ie coratgrrre 0.54 0.00270 2.57 
OP Oia ee 0.49 0.00294 2.53 
ER eer Roe 0.43 0.00301 2.52 one poe ee 
Oo ee eh ewan 0.41 0.00328 2.48 0.003 0.0000240 4.62 
O10). Fok ae soe 0.40 0.00360 2.44 AOE arnt or 
LOS nae 0.39 0.00390 2.40 
2D ee Wee eee 0.27 0.00540 2.27 


* roo per cent ionization taken as complete ionization into Ht and H2BO,’. 


IONIZATION OF ACIDS AND BASES 85 


TABLE VIL 
TARTARIC ACID * Citric Acrip t 
Moles 

of acid Percent Moles Ht Per cent Moles H* 

per liter ionized perliter pHvalue ionized per liter pH value 
ict) Coe ee ee 65.3 0.0007 15 60.2 0.0006 S22 
Oy ea ee 51.0 0.0010 3.00 47.4 0.0009 3.05 
2) 7 ae ee 43.0 0.0013 2.89 39.8 0.0012 2.92 
Lt) 39.0 0.0016 2.80 36.0 0.0014 2.85 
aoe vivid ed + 35.5 0.0018 2.74 33.1 0.0017 277; 
OOO Meteo ks <o.8's 33.0 0.0020 2.70 30.8 0.0018 2.74 
Bae os Feiss 6 31.0 0.0022 2.66 28.9 0.0020 2.70 
00S Gini se ag va 30.0 0.0024 2.62 27.6 0.0022 2.66 
EES) Se eee 28.0 0.0025 2.60 25.9 0.0023 2.64 
WET) Gs Sah ee 27.0 0.0027 2.57 25.0 0.0025 2.60 
NO pee ee A 19.5 0.0039 2.41 18.3 0.0037, 2.43 
Oe he ees os 16.5 0.0050 2.30 15.5 0.0047 2.33 
CAIs ©, x elaiin. os 14.5 0.0058 2.24 13.695 0.0055 2.26 
EE a0 ana 13.1 0.0066 2.18 12.5 0.0063 2.20 
MO aes cess 52:2 0.0073 2.14 II.5 0.0069 2.16 
DR a dow s+ 11.4 0.0080 2.10 10.7 0.0075 212 
Pe Ae os Fa ks 10.9 0.0087 2.06 10.1 0.0081 2.09 
eT Coe a 10.2 0.0092 2.04 9.5 0.0086 2.07 
ia a Re ea 9.9 0.010 2.00 9.1 0.009 2.04 
aaa A ates ex 3 7st 0.014 1.85 6.1 0.012 1.92 
Crates core 0:8 at? 0.017 177 Ag 0.014 1.85 
ae yd sk ys 4.9 0.020 1.70 4.0 0.016 1.80 
Pee eee ek ew ais ac Ag 0.021 1.68 3.5 0.018 1.74 
Trt oe ie ay 0.022 1.66 2:3 0.019 1.72 
(it ) Ree ee 3.5 0.025 1.60 3.0 0.021 1.68 
TS yee See eee 3.2 0.026 1.58 2.9 0.023 1.64 
Ge ree 3.0 0.027 1.57 2.8 0.025 1.60 
ee ie ee 2.9 0.029 1.54 20 0.027 1.57 
ap, et pe ait 0.042 1.38 1.8 0.036 1.44 


* 100 per cent ionization taken as complete ionization into H+ and HC,H,0O,’. 
+ 100 per cent ionization taken as complete ionization into H+ and H2CgH;0,’. 


86 THE CHEMISTRY OF LEATHER MANUFACTURE _ 


TABLE VIII. 
OxaLic Acip* SALICYLIC ACID 

Moles ' ‘tee 
of acid Per cent Moles Ht Percent MolesHt — 
per liter ionized perliter pHvalue ionized perliter pH val 
O.00T ears ieee ss sake wie % Beue. 62.0 0.0006 
0.002..... Gop a Tee poe pone 51.0 0.0010 
0.003..... Ree er Beer eed seo 44.5. 0.0013 
0.004...... etek SO 0.0038 2.42 40.0 0.0016 — 
OOS eta. ee Ne COs. 0.0047 2.33 37.0 4% ..0.001g 
0.006..... ay PE ep 0.0055. 2.26 34.5 0.0021 
O,007 22% = ee re eae sa 0.0063 2.20 32.0 0.0022 
O.005 sees eee 89.0 0.0071 2.15 30.5 0.0024 
0.000 awe ak OG. O.0070.= “2 2.00 29.0 0.0026 
DOIG oie ry ee COTO 0.0087 2.06 7 Be 0.0028 . 
O.0TOTae. ca: baqorene bees Wet Be Sey 24.0 0.0040 
0.020 Reis See cea 670) 0.0158 1.80 creak Pr es 
O80 Ras cite een i ae 0.0221 1.66 Pa gies 


* 100 per cent ionization taken as complete ionization into H+ and HC,O,’. 


Moles 
of base 
per liter 


PoONIZATION OF ACIDS AND BASES 


TABLE IX. 


PotrassiIuM HypbROXIDE 


Per cent Moles OH’ 


ionized __ per liter 
Tee 100.0 0.001 
nh i 100.0 0.002 
eet. 100.0 0.003 
af one 100.0 0.004 
ee 100.0 0.005 
Aaya 100.0 0.006 
Be oie 100.0 0.007 
eae 100.0 0.008 
RR 99.9 0.009 
Ata 99.9 0.010 
ac 99.3 0.020 
‘aig 08.7 0.030 
fob ve 97.9 0.039 
Telos 97.3 0.049 
ae 96.7 0.058 
Riedie 96.2 0.067 
Soe A 95.8 0.077 
pew tt: 95.3 0.086 
See 95.0 0.095 
Te 92.2 0.184 
See 90.1 0.270 
tae 88.8 0.355 
“pes 87.6 0.438 
ee 86.3 0.518 
ter 85.0 0.595 
ea 84.3 0.674 
Shee 82.8 0.745 
ore 81.9 0.819 


aii sk 66.3 1.326 


pH value 


11.00 
11.30 
11.48 
11.60 
11.70 
11.78 
11.85 
11.90 
11.95 


12.00 
12.30 
12.48 
12.59 
12.69 
12.76 
12.83 
12.89 
12.93 


12.98 
13.26 
13.43 
13.55 
13.64 
13.71 
13.77 
13.83 
13.87 


13.91 


14.12 


87 


SopluM HyproxIDE 


Per cent Moles OH’ 


ionized 


100.00 
I00.0 
100.0 
100.0 
100.0 
100.0 
100.0 
99.9 
99.7 


99.5 
97.9 
96.8 
96.0 
95.3 
04.7 
94.1 
93.7 
93.2 


92.9 
89.8 
87.0 
85.3 
83.5 
81.9 
80.4 
79.2 
77-7 


76.6 
57.0 


per liter pH value 


0.001 
0.002 
0.003 
0.004 
0.005 
0.006 
0.007 
0.008 
0.009 


0.010 
0.020 
0.029 
0.038 
0.048 
0.057 
0.066 


0.075 
0.084 


0.093 
0.180 
0.261 
0.341 
0.418 
0.491 
0.563 
0.634 
0.699 


0.766 


1.140 


11.00 
11.30 
11.48 
11.60 
11.70 
11.78 
11.85 
11.90 
11.95 


12.00 
12.30 
12.46 
12.58 
12.68 
12.76 
12.82 
12.88 
12.92 


12.97 
13.26 
13.42 
13.53 
13.62 
13.69 
13.75 
13.80 
13.84 


13.88 


14.06 


88 THE CHEMISTRY OF LEATHER MANUFACTURE 


TABLE X. 
AMMONIUM HyproxIDE BartuM HyproxIpeE * 
Moles 

of base Per cent Moles OH’ Percent Moles OH’ 
per liter ionized __ per liter pH value ionized jperliter pH value 
O00 Tete tea 12.52 0.00013 10.11 96.0 0.0010 11.00 
O02 oie oan 8.90 0.00018 10.26 95.0 0.0019 11.28 
0.009 Sis aw ee es 7.44 0.00022 10.34 04.0 0.0028 11.45 
O.00d 56.5, Sat eer 6.48 0.00026 10.42 93.0 0.0037 11.57 
CLOOSTe. aes 5.82 0.00029 10.46 92.0 ~ 0.0046 11.66 
C.0062 ig? ates 5.33 0.00032 10.51 91.3 0.0055 Higa 
D007 E fo pe trate 4.93 0.00035 10.54 91.0 0.006 11.78 
OO089 Fa O.et 4.62 0.00037 10.57 90.5 0.007 11.85 
C000: a tees 4.37 0.00039 . 10.59 90.0 0.008 11.90 
UT) See he Pathe 4.15 0.00042 10.62 88.4 0.009 11.95 
O02) Tarte ca a 2.96 0.00059 10.77 86.0 0.017 12.23 
DO s 370 as ok 2.42 0.00073 10.86 82.8 0.025 12.40 
OOF Bad Sao 2.12 0.00085 10.93 81.0 0.032 12.51 
Oa hess ee 1.88 0.00094 10.97 80.0 0.040 — 12.60 
QAM sice 2 ace oe ere 0.00103 II.01 ae ee Accmeae 
0.07% 432 ines a 1.59 O.0OI II 11.05 
O.OR sib. snip 1.49 0.00119 11.08 
O008cmt oss ats 1.40 0.00126 II.10 
Ope ae seca 1.33 0.00133 II.12 
OAs een Sr. 0.94 0.00188 11.27 papi Secor srhahe 
OS ents cere ea ae 0.77 0.00231 11.36 fee eae: cheek 
GALe ices ‘0.07 0.00268 ETAZ 
Gowers we 0.60 0.00300 11.48 ie fpr oeile Pere 
ON fd oe 0.55 0.00330 11.52 Bee se rere 
Oeras ik cicae oe 0.50 0.00350 11.54 eae meee 4 sate 
D.Gweak, pee ¥, 0.47 0.00376 11.58 Sats i ae pane 
O.GRh ae Sues 0.45 0.00405 11.61 ya tats 
TOit : csntte oes 0.42 0.00420 11.62 aes Baise fee 
2O ees ae te 0.30 0.00600 11.78 sack eee gates 


* 100 per cent ionization taken. as complete ionization into BaOH+ and OH’. 
_ Note: Where figures for calcium hydroxide are desired, it is suggested that those for 
barium hydroxide be used. 


IONIZATION OF ACIDS AND BASES 89 


Effect of Added Salts. 


The figures given in the tables are for pure solutions of the acids 
or bases. The addition of sodium chloride, or other neutral chlorides, 
tends to increase the hydrogen-ion concentrations of acids 1% 14 15 16 
and the hydroxide-ion concentration of bases.1* Neutral sulfates, on 
the other hand, tend to decrease the hydrogen-ion concentrations of 
acids. . 


+1,5 


+1,0 


+0,.5 


Log [H*] 


=1,0 


1 2 3 4 
Moles of Salt per Liter 


Fic. 39.—Effect of concentration of various salts upon the hydrogen-ion con- 
centration of tenth-normal hydrochloric acid solution. 


This contrasting effect of chlorides and sulfates on the hydrogen- 
ion concentrations of solutions of sulfuric and hydrochloric acids was 
shown by Thomas and Baldwin.*’ Their results for 0.1 normal acids 
are shown in Figs. 39 and 4o. In each case a solution of acid was 


18 Poma, Z. physik. Chem. 88 (1914), 671. 

14 Harned, J. Am. Chem. Soc. 37 (1915), 2460. 

15 Fales and Nelson, ibid., 37 (1915), 2769. 

16 Thomas and Baldwin, J. Am. Leather Chem. Assoc. 13 (1918), 248. 

17 Contrasting Effects of Chlorides and Sulfates on the Hydrogen-Ion Concentrations of 
Acid Solutions. A. W. Thomas and M. E. Baldwin. J. Am. Chem, Soc, 41 (1919), 1981. 


go THE CHEMISTRY OF LEATHER MANUFACTURE 


mixed with a solution of salt and diluted to 100 cubic centimeters so 
that the final concentration of acid was 0.1 normal and that of the salt — 
the concentration whose effect was being studied. The hydrogen-ion 
concentrations were measured, by means of the hydrogen electrode, 
two days after the solutions were made up. 


Log (n*] 


Moles of Salt per Liter 


Fic. 40.—Effect of concentration of various salts upon the hydrogen-ion 
concentration of tenth-normal sulfuric acid solution. 


When the chlorides are arranged in order of their ability to increase 
the hydrogen-ion concentration, the following series is obtained: 


KCI = NH,Cl< NaCl LiCl< BaCh = Maw 


But this is also the order of increasing degree of hydration, or the 
number of molecules of water combined with the individual cations at 
infinite dilution. Poma‘** found that chlorides increase the hydrogen- 
ion concentrations of hydrochloric acid solutions in the following 
order : 


RbCI<KCl< LiCl<CaClh,< MgCl, 


IONIZATION OF ACIDS AND BASES ot 


In extending the work of Thomas and Baldwin, Wilson ** pointed 
out that one of the remarkable features of their results is that when 
the logarithm of the concentration of hydrogen ion is plotted against 
the concentration of added salt, in the case of the alkali chlorides, the 
curves are apparently straight lines, of the general formula 


log [H"] = log a+ bm 


where b is a constant, a the hydrogen-ion concentration when no salt 


Log [H*] 


1 2 3 4 
Moles of NaCl per Liter 


Fic. 41.—Effect of concentration of sodium chloride upon the hydrogen-ion con- 
es centration of various strengths of sulfuric acid solution. 


is present, and [H*] the hydrogen-ion concentration in the presence of 
m moles per liter of salt. 

It was also shown that this equation is independent of the strength 
of the acid solution, the value for b depending only upon the kind of 
alkali chloride added. Curves showing the effect of adding sodium 
chloride to four different concentrations of sulfuric acid are shown 
in Fig. 41. Apparently the curves are not only straight lines, but all 
four have the same slope, the average value for b being 0.205. 

The addition of 4 moles per liter of sodium chloride raises the 
hydrogen-ion concentration of 0.1 molar hydrochloric acid to 0.44 


18 Hydration as an Explanation of the Neutral Salt Effect. J. A. Wilson. J. Am. Chem, 


92> THE CHEMISTRY OF LEATHER MANUFPACTERe 


mole per liter, which can be accounted for only on the assumption that 
more than three-quarters of the water present has ceased to play the 
role of solvent. The hydration theory assumes that this is brought 
about by the water combining with the salt. 

If the rise in hydrogen-ion concentration is due to the removal 
of water by the added sodium chloride, it should be possible to de- 
termine the degree of hydration of the salt at any concentration from 
hydrogen-ion measurements. Assuming this to be so, we should 
reason as follows: From the above equation, log([H*]/a) = bm. But 
[H"]/a is the factor by which the acid concentration has been multi- 
plied by adding m moles per liter of salt. Let w represent the total 
number of moles of water, free or combined with salt, in 1 liter of 
solution containing m moles of salt. The moles of free water then 
equal wa/[H"] and the moles of water combined with one mole of 
salt equal (w/m) X (1—a/[H’]). Calling this latter value h, we 
have 

h = w(1 — 107™) /m. 


From this, hydration values can be calculated for any concentration 
of salt. For infinite dilution of salt, the expression becomes greatly 
simplified, for 

Limit | 

HG ch ee 


But at infinite dilution zw = 55.5 and hence 
pipes Bote! 9 


The calculated number of molecules of water combined with one mole- 
cule of sodium chloride at infinite dilution would thus be 128 x 0.205 
or 20.2, which is in striking agreement with the value 26.5 obtained 
by Smith ** from a very different type of measurement. Calculations 
of the degrees of hydration at infinite dilution of the chlorides of 
potassium, ammonium, and lithium made from the equation h = 128b 
also agreed fairly well with Smith’s corresponding values. 

A means is thus afforded to calculate the change of pH value 
that will be produced by the addition of a neutral chloride to an acid 
solution. Let I represent the pH value of the acid solution containing 
no salt, which may be found in the preceding tables. Let F be the 
pH value after the addition of m moles per liter of salt and H he 
the number of molecules of water combined with one molecule of salt 
at infinite dilution. Then 


F = I—0.0078Hm. 


The use of this equation does not depend upon the validity of the 
theory. The measurements of Thomas and Baldwin show that it may 
be used for the addition of chlorides to sulfuric and hydrochloric 
acids by substituting the following values for H: 


19 A Method for the Calculation of the Hydration of the Ions at Infini iluti 
McP. Smith. J. Am, Chem, Soc. 37 (1915), 722. oe 


meniZzellION OF ACIDS AND BASES 93 


potassium chloride 15 
ammonium chloride 15 
sodium chloride 20 
lithium chloride 35 
barium chloride 50 


The effect of adding sulfates cannot, however, be attributed to 
hydration, since they decrease the hydrogen-i ion concentrations of acid 
solutions. Their action is probably due to the formation of addition 
compounds complicated by hydration effects. For the hydrogen-ion 
concentrations of sulfuric and hydrochloric acid solutions containing 
neutral sulfates, reference should be made to the original papers of 
Thomas and Baldwin. 

‘For the degrees of ionization of a large number of different salts 
at various concentrations, the reader is referred to page 35 of the 
recent book of Kraus.” 

A skin is subjected to liquors of widely different pH value in 
passing through the tannery. From a lime liquor having a pH value 
of 12.5 it may pass into a bate liquor with a pH value of 7.5, then 
into a pickle liquor of pH —1.5, then into a chrome liquor whose 
pH value is rising from 3 to 4, and then into a fat liquor at pH = 9. 
Or, in vegetable tanning, the skin may pass from the bate liquor to a 
tan liquor whose pH value may be anything from 2.5 to 5.5, de- 
pending upon the method of operation of the yards. But in spite of 
the wide variation in pH value to which the skin is subjected in 
passing through the tannery, the processes are all sensitive to com- 
paratively small variations in pH value unless each variation is 
compensated by corresponding changes in the process itself. » 


20 The Properties of Electrically Conducting Systems. C., A. Kraus. Chemical Catalog 
Co., New York. Se baste 


_ Chapter 5. 


Physical Chemistry of the Proteins. 


The physical chemistry of the proteins is one of the foundations 
upon which leather chemistry is built, but until comparatively recently 
our knowledge of the chemical reactions of the proteins was hardly 
sufficient to permit of quantitative treatment. Proteins did not seem 
to show the stoichiometric relations of orthodox physical chemistry to 
earlier investigators because they failed to recognize the full number 
of phases existing in a given system and the necessity for making 
measurements at definite hydrogen-ion concentrations. ) 

The way for the quantitative development of the physical chemistry 
of the proteins was paved by the appearance of Donnan’s theory of 
membrane equilibria, which was applied by Procter? to the swelling 
of gelatin and further developed by Procter and Wilson * into a quantita- 
tive theory of the swelling of protein jellies. In an extensive series 
of researches, Loeb * has extended this work to include also the osmotic 
pressure, viscosity, stability, and electrical potential differences of 
protein systems as well as a general theory of colloidal behavior. This 
valuable work is now available in book form® and should be consulted 
as having an important bearing upon leather chemistry. 

It will be shown in this chapter that proteins conform to the classical 
laws of physical chemistry and that their reactions are indicated by. 
well established principles. Donnan’s theory forms the logical start- 
ing point for this presentation. A good discussion of Donnan’s theory 
is given in Lewis’ Physical Chemistry ;* we have extended it in this 
chapter to a consideration of the effects of valency. 


Donnan’s Theory of Membrane Equilibria. 


This theory deals with the equilibria resulting from the separation 
by a membrane of two solutions, one of which contains an ionogen 
having one ion that cannot diffuse through the membrane, which is 
permeable to all other ions of the system. As an example, Donnan 


1F.G. Donnan. Z, Elektrochem. 17 (1911), 572. 
Serve of Dilute Hydrochloric Acid and Gelatin. H. R. Procter. J. Chem. Soc. 
105 (1914), 313. 
( Boe Acid-Gelatin Equilibrium. H. R. Procter and J. A. Wilson. J. Chem. Soc. 109 
1916), 307. 
4J. General Physiol., 1918-1922. 
y hei and the Theory of Colloidal Behavior. Jacques Loeb. McGraw-Hill Book Ca.; 
ew York. 
- A System of Physical Chemistry, Vol. IT, Thermodynamics, pp. 275-86. Longmans, 
Green & Co., London, 


94 


feet a CHEMISTRY OF THE PROTEINS 95 


takes an aqueous solution of a salt NaR, such as Congo red, in contact 
with a membrane which is impermeable to the anion R’ and the non- 
ionized salt, but will allow Na* or any other ion to pass freely through 
it. The membrane separates the Congo red solution from an aqueous 
solution of sodium chloride, which will diffuse from its Solution II 
into the Solution I of NaR. When equilibrium is established, if a 
small virtual change is made reversibly at constant temperature and 
volume, the free energy will remain unchanged; that is, no work will 
‘be done. The change here considered is the transfer of dn moles of 
Na* and Cl’ from II to I. The work, which equals zero, is 


[Na*] 11 (Sauces 
NS alt + dn.RT.log (eur == 0, 
whence ey Cry = [Na |r X< [Cl’]1. 


(The brackets indicate concentration in moles per liter.) 


Equilibrium will be established only when the product of the concentra- 
tions of Na* and Cl’ has the same value on both sides of the membrane. 

This equation of products, simple though it may appear, is of such 
fundamental importance in the quantitative development of leather 
chemistry that any doubt as to its validity should be dispelled at the 
outset. The derivation of the equation need not involve the use of 
thermodynamics, since it can readily be visualized. In passing from 
one phase to the other, the oppositely charged ions must move in pairs, 
since they would otherwise set up powerful electrostatic forces that 
would prevent their free diffusion. For this reason a sodium or a 
chlorine ion striking the membrane alone could not pass through it. 
But, since the membrane is freely permeable to both Na* and Cl’, when 
two oppositely charged ions strike the membrane together, there 1s 
nothing to prevent them from passing through into the solution on the 
opposite side. The rate of transfer of these ions from one solution 
to the other depends, therefore, upon the frequency with which they 
chance to strike the membrane in pairs, which is measured by the 
product of their concentrations. At equilibrium the rate of transfer 
of Na* and Cl’ from Solution II to Solution I exactly equals the rate 
of transfer of these ions from Solution I to Solution II, from which 
it follows that the product of the concentrations of these ions has the 
same value in both solutions. 

It is interesting now to note the effect of complicating the system 
by the introduction of another salt, such as KBr. Following the same 
line of reasoning, it will be evident that equilibrium will be established 
only when the product [K*] X [Br’] has the same value in both solu- 
tions, and the same is true for the products [K*] & [Cl’] and 
[Nat] X [Br’]. In fact, with any number of mono-monovalent. iono- 
gens present in the system, the product of the concentrations of any 
_pair of diffusible and oppositely charged ions will have the same value 
in both solutions. 

Introducing polyvalent ions into the system makes the equation 


96 THE CHEMISTRY OF LEATHER MANUFACTURE 


of products but very little more complicated. When a polyvalent ion 
strikes the membrane, it will pass through only when an equivalent » 
number of ions of opposite sign strike the membrane at the same time 
and pass through with it. The rate of transfer of any dissociated 
ionogen from one solution to the other is evidently determined by 
the product of all the ions required to produce the undissociated ionogen. 
At equilibrium, this product will have the same value in both solutions. 
It, for example, the system contained the ions Nat and SO”,, then 
the product [Na*] X [Na*] x [SO”,], or [Nat]? x [SO”,], would 
have the same value on both sides of the membrane, at equilibrium. 
The impermeability of the membrane to the anion R’ causes an 
unequal distribution of ions between the two solutions. In Solution II 
of the simple system including only the ionogens NaR and NaCl, let 


x — [Nat esi Gah 


In Solution I let ye] tee 
and Zia) ee 
whereupon [Nat] =y +z. 


The equation of products may then be written 
x? = y(y +2)... 


But here we have the product of equals equated to the product of 
unequals, from which it is apparent, mathematically, that the sum of 
the unequals is greater than the sum of the equals, or that 


2y + z>2x. 


The reasoning thus indicates that the concentration of diffusible 
ions in Solution I, at equilibrium, is greater than in Solution II, and 
this has been shown to be true in numerous experiments. If we let 
the excess of diffusible ions of Solution I over Solution II be repre- 


sented by e, then 2y+z=e2x-+e, 
or x=y+ a/ ey, 


which shows us further that + is greater than y or that the concentration 
of ionized sodium chloride is greater in Solution II than in Solution I. 
The added sodium chloride does not distribute itself equally through- 
out both solutions, but, at equilibrium, it is the more concentrated 
in Solution II. : | | 

The different distribution of ions in the solutions at equilibrium 
gives rise, not only to a difference in osmotic pressure, but also to an 
electrical difference of potential across the membrane. Donnan de- 
rived the equation for this potential difference by the following 
thermodynamic reasoning. : 

In the system just described, let xy be the potential, for positive 
electricity, of solution I and xyz that for Solution II. Let the ex- 


PHYSICAL CHEMISTRY OF THE PROTEINS 97 


tremely small quantity Fdn of positive electricity be transferred isO- 
thermally from II to I. In this virtual change of the system from 
equilibrium, the following work terms must be considered: the change 
in free electrical energy represented by Fdn (ayy — 1) and the simul- 
taneous transfer of pdn moles of Na* from II to I and of qdu moles 
of Cl’ from I to II, where p and q are the respective transport num- 
bers of the ions, and hence p+q=1. The maximum osmotic work 
of operation of this transfer of ions is represented by the expression 


[Na‘]11 [Cl] 1 
pdnRT . log (Na‘|r + qdnRT .log in 


But, since the system is in equilibrium, the electrical virtual work must 
balance the osmotic virtual work, or 


Fdn (xt — arr) = pdnRT. log alas +. qdnRT .log Car 
II 


[Na']t 


| Na*|11 Ge bevy ee ev * 
eo Meigs (Clit sy andp-+q=1. Letting E= ay —a]1, 
we have 


rE 
oS log = volts. 
This is an equation of fundamental importance in the theory of the 
mechanism of many reactions involved in leather making. 

It will now be shown that this equation is still valid when other 
ions of any valency are added to the system. Consider the general 
case where an ionogen yielding the ion M** of valency a is added. By 
applying the above line of reasoning to the potential difference pro- 
duced by the unequal distribution of the ions of the added ionogen 
between solutions I and II, we arrive at the equation 

RT | [M**Jar 
— —— .log = 
B= or °8 [MF] 
where n =a, the valency of M**. But it is evident from the equation 
of products that 


[Maya >< [CV ]er = [Mar & [Cl] Arr 
and that eee ier x [CV \*1 = [Na Jaa [CEP 
from which it is apparent that 

[M**]rz _ [Na*]*rr _ x* 
UR NEM ia et 
Therefore 
Rel 2 he Le x 


E=— .log — = — .log — 


a Lire y? F y 


os THE CHEMISTRY OF LEATHER MANUFACTURE 


At equilibrium, the unequal distribution of the added ionogen between 
solutions I and II produces exactly the same potential difference as the 
unequal distribution of sodium chloride. Although the addition of any 
ionogen must produce a change in the measured potential difference, 
by disturbing the equilibrium, all ionogens present when equilibrium is 
again established are producing the same potential difference, regardless 
of valency. The potential difference can thus be calculated from the 
determination of the distribution of only one kind of ion between the 
two solutions. 

The complexity of systems, such as those just described, is due to 
the fact that the membrane prevents the diffusion of one kind of ion 
from one phase to the other. A similar set of conditions is brought 
about whenever one of a number of ions of a system is prevented from 
diffusing from one phase to another, which is true for every basic 
tannery process. When skin protein is brought into equilibrium with 
various tannery liquors, the diffusion of the protein ions is prevented, 
not by a membrane, but by their own forces of cohesion. This will be 
made clear in discussing the swelling of proteins. 


Swelling of Protein Jellies. 


When a strip of dry gelatin is soaked in water, it swells by absorb- 
ing water, increasing in volume from 5 to Io times, depending upon the 
temperature of the water and the quality of the gelatin. With increas- 
ing concentration of acid, or alkali, the swelling increases to a maximum 
and then decreases. The property of swelling in aqueous solutions 
appears to be common to all proteins under conditions such that they 
do not pass directly into solution. The swelling caused by acids and 
alkalies is generally counteracted by the addition of neutral salt or by 
increasing the concentration of acid or alkali sufficiently. 

While attempting to arrive at a rational explanation of the molecular 
mechanism of tanning, Procter was continually confronted by the neces- 
sity of first explaining the mechanism of swelling and to him belongs the 
credit of being the first to recognize the almost complete dependence of 
the science of leather chemistry upon the theory of swelling. In 1897 
he started an investigation? of the swelling of gelatin in solutions of 
acids and salts which has culminated in the Procter-Wilson theory of 
swelling. 

Procter’s general method of experimentation was as follows: Sheets 
of thin, purified bone gelatin were cut into portions containing exactly 
I gram each of dry gelatin. A portion was put into each of a series 
of stoppered bottles containing 100 cubic centimeters of hydrochloric 
acid of definite concentration. A fter 48 hours, which was shown to be 
sufficient for the attainment of practical equilibrium, the remaining solu- 
tion was drained off and titrated with standard alkali. The gelatin 
plates were quickly weighed and the volume of solution absorbed was 
calculated from the increase in weight of the plates. The swollen 


7 Action of Dilute Acids and Salt Solutions upon Gelatin. H, R, Procter. K lloidchem. 
Bethefte (1911); J, Am, Leather Chem, Assoc. 6 (1911), a rocter. Kolloidchem 


Meets CAl CHEMISTRY OF THE PROTEINS 99 


gelatin was then put back into the bottles and covered with enough dry 
sodium chloride to saturate the solution which had been absorbed by 
the gelatin. This caused the gelatin to contract and give up the ab- 
sorbed solution. After 24 hours, when equilibrium was again estab- 
lished, the solution expelled by the salt was drained off and titrated to 
determine the amount of free acid which had been absorbed by the 
gelatin. A small amount, usually about 1 cubic centimeter, of solution 
always remained unexpelled by the salt and, although not strictly true, 
this was assumed to have the same concentration of free acid as the 
portion expelled, due allowance being made for the increase in volume 
of solution due to saturating it with salt. The acid still unaccounted 
for was assumed to be combined with the gelatin base. i 

A further set of checks was obtained by dissolving the gelatin, dehy- 
drated by treatment with salt, in warm water and titrating with standard 
alkali, using both methyl orange and phenolphthalein, the former indi- 
cating the free acid left in the jelly and the latter the total, including 
the acid combined with the gelatin base, which was obtained by dif- 
ference. 

Experimental values for the volume of solution absorbed by the 
gelatin, the free acid left in the external solution, the free acid in the 
jelly, and the acid combined with the gelatin base are shown in Table 
XI and in Figs. 42 and 43. These were taken from the table on page 
317 of Procter’s paper, The Equilibrium of Dilute Hydrochloric Acid 
and Gelatin. In plotting the results, the concentration of gelatin 
chloride is taken as the difference between the concentrations of total 
chloride and free HCl in the jelly. The calculated values given along 
with the experimental ones will be discussed later in connection with 
the theory. 


The Acid-Protein Equilibrium. 


Procter recognized that gelatin combines with HCI forming a highly 
ionizable chloride and that the resulting equilibrium is a special case of 
the membrane equilibria described by Donnan. Instead of tracing the 
development of the theory of swelling from Procter’s earliest work to 
its present status, it will simplify matters to present the theory from 
the deductive reasoning furnished later by Wilson and Wilson.® They 
set out to prove that the entire equilibria can be determined quantita- 
tively from the orthodox laws of physical chemistry on the simple 
assumption that gelatin, or any protein, combines with hydrochloric 
acid to form a highly ionizable chloride. It seemed that success in this 
would furnish substantial proof of the correctness of the theory. 

In order to make the reasoning general, let us consider the hypo- 
thetical protein G, which is a jelly insoluble in water, is completely 
permeable to water and all dissolved ionogens considered, is elastic and 
under all conditions under consideration follows Hooke’s law, and com- 


8 J. Chem. Soc. 105 (1914), 313. 


3 
® Colloidal Phenomena and the Adsorption F la. J. A. and W. H. Wilson. 
Chem, Soc. 40 (1918), 886, phon seerpulas Py an Wilson. J. Am. 


ST. BONAVENTURE 


CHEN. ISTRY 
100 THE CHEMISTRY OF LETT DRA RY iN UFACTURE 


bines chemically with the hydrogen ion, but not the anion, of the = 
HA according to the equation 


[G] x [H*] = K[GH*]. (1) 


In other words, the compound GHA is completely ionized into GH* 
and A’. 

Now take one millimole of G and immerse it in an aqueous solution 
of HA. The solution penetrates G, which thereupon combines with 
some of the hydrogen ions, removing them from solution, and conse- 
quently the solution within the jelly will have a greater concentration 
of A’ than of H*, while in the external solution [H*] is necessarily 
equal to [A’]. The solution thus becomes separated into two phases, 
that within and that surrounding the jelly, and the ions of one phase 
must finally reach equilibrium with those of the other phase. 

At equilibrium, in the external solution, let 


x= [H*] = [A’] 
and in the jelly phase let 


y = [H*] 
and Params) Ole bs 
whence [A’] =y+z. 


It should be remembered that the brackets indicate concentration in 
moles per liter. 

It is apparent from Donnan’s line of reasoning, given earlier in the 
chapter, that the product [H*] * [A’] will have the same value in the 
external solution as in the jelly phase at equilibrium, or that 


x? = y(y +2). (2) 
‘ As was pointed out above, it is evident from equation (2) that 
2y +z> 2x 
or 2y-+z=2x+e (3) 


where e is defined as the excess of concentration of diffusible ions of 
the jelly phase over that of the external solution. Where any two 
variables are known, all others can be calculated, for from equations 
(2) and (3) we get the following: 


x=y+ Vey= Vy? +yz= (2? —e*)/ge. (4) 
¥ a= (2 Ae + 4x*)/2=> (2x + e— V/ 4ex + e?)/2 = 
(z— e)?/4e. (5) 
z= (x*—y*)/y = Vgex =e +2 Vey. (6) 
e = (x—y)*/y =z + 2y—2 Vy? yz =— 2x4 


Vi4xt 22, Bee 


PHYSICAL CHEMISTRY OF THE PROTEINS 101 


Since [A’] is greater in the jelly than in the surrounding solution, 
the negative ions of the colloid compound will tend to diffuse outward 
into the external solution, but this they cannot do without dragging 
their protein cations with them. On the other hand, the cohesive forces 
of the elastic jelly will resist this outward pull, the quantitative measure 
of which is e, and according to Hooke’s law 


ea ONE (8) 


where C is a constant corresponding to the bulk modulus of the protein 
and V is.the increase in volume, in cubic centimeters, of 1 millimole 
of the protein. 

Since we have taken 1 millimole of G, 


beater [GH*] —=1/(V +a) 
or [G] = 1/(V -+-a) —z (9) 


where a is the initial volume of 1 millimole of the protein. 


From (1) and (9) 


z=y/(V +a)(K+y) (10) 
and from (6) and (8) 
z—CV+2WvVCVy. (11) 
Now from (10) and (11) 
Oye aj(k + y)(CV +2 VCVy) —y=o (12) 


where the only variables are V and y. 

If the molecules or atoms of the protein are not themselves per- 
meable to all ions considered, the quantity a@ should not be taken as 
the whole of the initial volume of the jelly, but only as the free space 
within the original, dry jelly through which ions can pass. For our 
hypothetical protein, then, we shall consider the limiting case where 
the value of a is zero. This assumption in the case of gelatin intro- 
duces errors less than the probable experimental error because of the 
relatively large values for V over the significant swelling range. [qua- 
tion (12) thus reduces to 


V(K + y) (CV +2 VCVy) —y=o. (13) 


Knowing the values of the constants, K and C, we can plot the 
entire equilibrium as a function of any one variable. Procter and 
Wilson *® obtained the value K = 0.00015 for the sample of gelatin 
used in their experiments by adding successive portions of standard 
HCl to a dilute solution of the gelatin and noting the corresponding 
rises in hydrogen-ion concentration. The difference between the con- 
centration of hydrogen ion that would have been. found upon adding 
the acid to pure water and that actually found by adding it to the same 


~The Acid-Gelatin Equilibrium, Joc. cit. 


1o2 THE CHEMISTRY OF LEATHER MANUFACTURE 


volume of gelatin solution was taken as the amount of acid combined 
with the gelatin, or as the value of [GH'*] in equation (1). Substitut- 
ing any two sets of determinations of [GH*] and [H*] in equation (1) 
and solving the resulting equations simultaneously, the value of “ can 
be found. 

C was obtained by substituting experimental values for V Aaa e in 


Procter's observed 
results: 


X= total chloride, 
e = free HCl, 
0.251 O= gelatin chloride 


. Continuous 
0,15 lines represent 


calculated values, 


0,10 


Concentrations in Jelly (moles per liter) 


0,05 


0.10 0.20 0.30 
{uH*] in External Solution 


Fic. 42. —Observed and calculated values for the distribution of HC1 in the 
system Gelatin-HCl-Water. 


equation (8). It was found to vary with the temperature and with 
the quality of the gelatin, but had the value 0.0003 for the sample of 
gelatin used by Procter and at the temperature of his experiments, 18° C, 

In order to compare calculated values for V with experimental 
determinations of the increase in volume of I gram of gelatin, it is 
necessary to know its equivalent weight. Procter originally regarded 
gelatin as a diacid base with a molecular weight of 839, but later work 
by Procter and Wilson showed that it should rather be regarded as 
acting as a monacid base, with an equivalent weight of 768, in acid 
solutions not sufficiently concentrated to cause decomposition. 768 


& 


PeystCAlL CHEMISTRY OF THE PROTEINS 103 


grams of gelatin combine with a limiting value of 1 mole of hydrochloric 
acid and the combination resembles that of HCl with a weak monacid 
base. For this reason we may use the value 768 as the equivalent 
weight of gelatin. As for the molecular weight of gelatin, no con- 
vincing figures have yet been produced and it may be questioned whether 


Continuous line represents 
calculated values. 


Circles represent Procter's 
observed results. 


Increase in Volume of 1 Gram of Gelatin (c.c.) 


0.10 0.20 0.30 
{H+] in External Solution 


Fic. 43.—Observed and calculated values for the degree of swelling of gelatin asa 
function of the concentration of hydrochloric acid. 


they would have any real value, if obtained. We look upon a plate of 
gelatin as a continuous network of chains of amino acids, there being 
no individual molecules, unless one wishes to look upon the plate of 
gelatin as one huge molecule. 

From equation (13) and the values of the constants given above, 
Wilson and Wilson calculated all of the variables of the equilibrium 
for gelatin and hydrochloric acid over the range covered by Procter’s 


104 THE CHEMISTRY OF LEATHER MANUFACTURE 


experiments. The important variables are shown in Table XI and in 
Figs. 42 and 43 along with Procter’s actual determinations. 

The agreement between calculated and observed values is absolute, 
within the limits of experimental error. For this reason Procter and 
Wilson regard their theory as proved, but, if further corroboration is 
desired, it can be found in the extensive researches of Loeb, some of 
which will be described later. Jt is worthy of note that no other theory 
of swelling has yet passed the stage of qualitative speculation. 


TABLE XI. 


At Equilibrium 
Cc. solution 
absorbed by I g. [ Total chloride} 
[HCl] V gelatin [HC1] in jelly in jelly 
Initial in Calcu- Calcu- Ob- Calcu- Ob- Calcu- Ob- 
[HCI] soln. lated lated served lated served lated served 


0.006 0.0011 o35 43.4 44.1 0.0001 0.0005 0.012 0.014 
0.008 0.0018 37.5 48.8 48.7 0.0002 0.0004 0.014 0.015 
0.010 0.0025 41.7 54.3 59.9 0.0004 0.0004 0.016 0.015 
0.010 0.0028 42.7 55.60 58.4 0.0004 0.0004 0.017 0.015 
0.010 0.0032 43.2 56.2 53-7 0.0005 0.0005 0.019 0.017 


0.015 0.0073 40.8 53.1 57-9 0.002 0.002 0.024 0.020 
0.015 0.0077 40.2 52.3 522 0.002 0.002 0.025 0.022 
0.015 0.0120 37.5 48.8 51.9 0.005 0.006 0.031 0.027 
0.020 0.0122 a4 48.6 4 Oy, 0.005 0.006 0.031 0.027 


0.025 0.0170 34.5 44.0 40.4 0.008 0.009 0.036 0.037 
0.025 0.0172 34.3 44.7 48.1 0.008 0.009 0.036 0.031 
0.050 0.0406 20.7 34.8 36.4 0.026 0.030 0.063 0.061 
0.050 0.0420 26.4 34.4 31.1 0.027 0.030 0.005 0.008 
aie 0.0576 24.0 212 34.0 0.041 0.043 0.082 0.079 
0.075 0.0666 23.0 29.9 27.9 0.049 0.050 0.092 0.095 
0.075 0.0680 22.8 29.7 29.1 0.050 0.053 0.0904 0.092 
0.100 0.0930 20.7 27.0 23.1 0.072 0.072 0.121 0.126 
0.100 0.09044 20.5 26.7 26.4 0.073 * 0.072 0.122 0.121 
ae 0.1052 19.8 25.8 29.8 0.083 0.085 0.134 0.128 
0.125 0.1180 18.9 24.6 24.4 0.095 0.090 0.148 0.148 
0.150 0.1434 17.9 233 24.0 0.118 0.118 0.174 0.173 
0.150 0.1435 17.9 23-3 24.2 0.118 0.118 0.174 0.172 


0.175 0.1685 17,3 22.2 23.5 0.141 0.138 0.200 0.200 
0.200 0.1925 16.3 21-2 20.6 0.164 0.161 0.225 0.229 
0.200 0.1940 16.2 alak 227 0.166 0.165 0.227 0.225 
0.200 0.1945 16.2 2I.1 22.1 0.167 0.164 0.228 0.226 
0.250 0.2450 15.1 10.7 20.2 0.213 0.210 0.279 0.281 
0.300 0.2950 14.0 18.2 20.0 0.261 0.260 0.332 0.332 


Other proteins which do not dissolve in cold water behave much like 
gelatin in respect to swelling, although they apparently have different 
values for the constants, K and C, as well as for equivalent weight. It 
is interesting to reason from the theory what differences in swelling 
would result from changes in the values of the constants. Since 
V = e/C, an increase in the value of C means a corresponding decrease 
in the degree of swelling. The effect of a change in the value of K, 
the hydrolysis constant of the protein, is shown in Fig. 44 for a fixed 
value of C. At K=o, the point of maximum swelling occurs at 


PHYSICAL CHEMISTRY OF THE PROTEINS 105 


x =o and has the value 1/7/C. As K increases in value, the point 
of maximum swelling decreases in value and occurs at increasing values 
for +. At K= oo, the point of maximum has the value zero and 
occurs at 7 = oo 

According to the theory, all monobasic acids should produce the 
same degree of swelling of gelatin for any fixed hydrogen-ion concen- 


The broken line represents the 


te) 
(eo) 


locus of all points of maximum 


@ 
(o) 


swelling for C = 0.0001, 


J 
o 


en) 
Oo 


Increase in Volume of 1 Millimole of Protein (c.c.) 


0.02 0.04 0.06 
(H*] in External Solution 


Fic. 44.—Family of swelling curves for proteins having the same bulk modulus, 
but different values for the hydrolysis constant. 


tration, under constant conditions, provided the gelatin salts formed are 
ionized to the same extent. It was generally thought that different 
monobasic acids produce different degrees of swelling, following the 
order of the well-known Hofmeister series of the ions, until Loeb 
pointed out that the earlier investigators, through failure to measure 
the hydrogen-ion concentration, had fallen into the error of attributing 
to the several acids effects caused merely by differences in hydrogen-ion 
concentration. He found, at a fixed value for x, that practically the 
same degree of swelling is produced by all monobasic acids, as well as 


106 THE CHEMISTRY OF LEATHER MANUFACTURE 


such acids as phosphoric and oxalic at concentrations at which they act 
as monobasic. 

The calculation of the degree of swelling of proteins in solutions of 
polybasic acids is not quite so simple as for monobasic acids. Suppose | 
that G were to combine with the hydrogen ion but not the anion of 
the polybasic acid H,A. Letting x represent the concentration of the 
polyvalent anion in the external solution at equilibrium, zg the concen- 
tration of the anion of the.gelatin salt, and y + zg the total concentration 
of anion in the jelly, it is evident from the reasoning given above that 


xt = y*(y +2) 
and, by inspection of this equation, we see that 
 (a+i1)x< (+ t)y4Z 
or that (a+1)x+e=(a+I1)y+z. 


The total concentration of diffusible ions is greater in the jelly than in 
the external solution by the amount e and swelling in degree directly 
proportional to e will result. It can readily be seen that as x increases 
from zero, without limit, e and the degree of swelling increase to a 
maximum and then decrease, approaching zero, for z has a limiting 
value since it cannot exceed the total concentration of gelatin. At 
4 =0, y=0, ande=o. As ~# increases without limit our equations 
approach the limiting relations 


eevee 
and (a+1)xte=(at+t1)y 


from which it is evident that +r = y and e =o. 

The extent of swelling by polybasic acids which combine as such 
with the protein will be considerably less than that caused by monobasic 
acids, as Loeb has shown, because fewer anions will be associated with 
equivalent weights of the protein. For example, for equivalent weights 
of gelatin sulfate and gelatin chloride, there would be only half as 
many sulfate ions as chloride ions. For very small values of +, we 
should therefore expect sulfuric acid to produce only half as much 
swelling as hydrochloric acid at the same hydrogen-ion concentration 
and this is actually the case. 


Repression of Swelling by Salts. 


The theory accounts quantitatively for the action of neutral salts 
in repressing the swelling of proteins by acid. In the system described 
above in which the protein G was immersed in a solution of HA, con- 
sider the addition of the mono-monovalent salt MN, neither of whose 
ions combine with G. At equilibrium, let the concentration of M* be 
represented by u in the external solution and by v in the jelly. It is 
evident from the general equation of products that the product 


* (LT H*y EM") ) CAS ae 


PHYSICAL CHEMISTRY OF THE PROTEINS 107 
will have the same value in both phases, or that 


fete (Yet vy + ¥ + 2) 


from which 


Bea (y -- v) 2 2(x- 4). 


Solving the two preceding equations simultaneously, we get 
e=—a(xtu)+ V4(x+u)? +2. 


Now, if the value of x» -+ u increases while remains constant, the 
value of e, and consequently the swelling, will decrease. The addition 
of MN to the system increases u and hence must cause a decrease in 
the degree of swelling, since it increases z only by causing a diminution 
of the volume of the jelly. 

It is important to recognize that the repression of swelling by salts 
does not depend upon any repression of ionization of the protein salt. 
The salt acts so as to lower the value of e, which is the measure of 
the force producing swelling. In some cases, the ionization may be 
repressed to some extent and this would assist in repressing the swell- 
ing, but in the case of gelatin chloride, the swelling is markedly re- 
duced long before there is any repression of ionization of gelatin chloride 
measurable by means of calomel electrodes. 


The Alkali-Protein Equilibrium. 


Proteins are amphoteric substances, reacting both as weak acids and 
as weak bases. In this respect, they retain the properties of the amino 
acids from which they are formed. Hydrated aminoacetic acid is 
capable of assuming either a positive or negative charge, or both, by 
ionizing as acid or base, or both, thus: 


H+ + ‘OOC.CH,.NH,. HOH = HOOC.CH,.NH,.HOH = 
HOOC.CH,.NH,.H* + OH’. 


The ionization constant of a protein as an acid may be represented as 
follows : 


LEE eq RO Genet 5 BS AE 
But foe (On| — K, or [H*)—K,/(0On'] 
from which [GH] x [OH’] =k[G’], where k = Ky/Ka. 


But this is essentially the same as equation (1) except for the fact 
that [H*] is replaced by [OH’]. It is thus apparent that proteins 
will behave in solutions of increasing concentration of alkali much 
as they do in solutions of acids so long as they undergo no chemical 
changes other than that of salt formation. Actually gelatin swells 
in alkaline solution to a maximum at a concentration of about 0.004 
mole of hydroxide ion per liter, above which the swelling diminishes. 


108 THE CHEMISTRY OF LEATHER MANUFACTURE 


In acid solution maximum swelling occurs at a concentration of 0.004 
mole of hydrogen ion per liter. 

The effect of valency is similar in both acid and alkaline solutions. 
Loeb found that the diacid bases calcium hydroxide and barium 
hydroxide give points of maximum swelling for gelatin only half as 
great as the monacid bases. For a given pH value, the amount of swell- 


TABLE XII. 
[ISOELECTRIC Pornts or SEVERAL ProrriNs IN TERMS oF —'LoG [H+] or pH Vatrue. 


— Log [H+] 
or pH value Reference 


CASEIN (COW). uubok tei oe ae a eee 4.6 I 

4.7 2 

4.7 3 
Gelatin ad saat? sine asi ape Seek Cee eee ae 4.6 4 

4.7 5 
Seriim sal bimiwyr sey c.me eae eee ee See 47 6 
perim globulin >A oes. 25. st cae ee eee ee 5.4 2 
Eee albumen «then)ofic: 22). cee ae 4.8 7 
Denatured :serum albumin, 3s... See 5.4 6 
Oxybemoriobing ius. 3042 wos eee eee 6.7 9 
Carbon monoxide hemoglobin................ 6.8 10 
Reduced (hemoglobin 2.5.05 tu. s cee ee 6.8 10 
Stroma globulins of blood corpuscles......... 5.0 8,9 
Reds blood cellsncas si iene oe eee 4.6 iL 
Yeast extract proteins, (globulin). ............ 4.6 14 
Csliaclin, «Ut uae tetas aaa en ee 9.2 2 
Edestin ae cckos ON pet tes ee 5.6 15 
Juberin >» (potata) fa, 2s can gue see ee eee approx. 4.0 12 
Carrot. protein)... seen ee ee A 4.0 12 
Lomato protein’ a; Soy tok ee ee ee % 5.0 12 
Nuacleiccacidii, S55 come e cs io eee ae a ie 2.0 13 

REFERENCES: 


1. Michaelis and Pechstein. Biochem. Z. 47 (1914), 260. 
2. Rona and Michaelis. Ibid. 28 (1910), 193. 

3. Loeb. J. General Physiol. 2 (1920), 577. 

4. Michaelis and Grineff. Biochem. Z. 41 (1912), 373. 

5. Loeb. J. General Physiol. 1 (1918), 39. 

6. Michaelis and Davidsohn. Biochem. Z. 33 (1911), 456. 

7. Sorensen. Compt-rendus trav. lab. Carlsberg, 12 (1915-17). 
8. Michaelis and Davidsohn. Biochem. Z. 41 (1912), 102, 

9. Michaelis and Takahashi. Jbid. 29 (1910), 439. 

10. Michaelis and Bien. Ibid. 67 (1914), 108. 

11, Coulter, J. General Physiol. 3 (1921), 309. 

12. Cohn, Gross and Johnson. Jbid. 2 (1919), 145. 

13. Michaelis and Davidsohn. Ibid. 39 (1912), 496. 

14. Fodor. Kolloid. Z. 27 (1920), 58. 

15. Michaelis and Mendelssohn. Biochem. Z. 65 (1954), 3; 


ing is determined by the valency of the ions of opposite sign to that of 
the protein ions rather than by the specific nature of the ions themselves. 

In alkaline solution the protein ion is negatively charged, while 
it is positively charged in acid solution. In a solution, originally 
alkaline, in which the hydrogen-ion concentration is gradually increased, 
there must be some point at which the protein becomes electrically 
neutral ; that is, where it has an equivalent number of positive and nega- 
tive charges. The hydrogen-ion concentration at which this occurs has 


meen CHEMISTRY OF THE PROTEINS 10g 


been called by Hardy! the isoelectric point of the protein. The iso- 
electric point of gelatin was found by Michaelis and Grineff 12 to lie 
at a pH value of 4.7 and this value has been repeatedly confirmed by 
Loeb and others. 

Thomas and Kelly ** determined the isoelectric point of collagen, 
or rather hide powder, by means of acid and basic dyes. Portions 
of hide powder were first wet with solutions of different pH values, 
then with solutions of basic fuchsin or Martius yellow, and finally 
washed with solutions having the same pH values as were used to wet 
the portions initially. The fuchsin left the hide powder deeply stained 
only at pH values greater than 5 and the Martius yellow only at 
values below 5, indicating pH = 5 as the isoelectric point of collagen. 

Porter ** observed that a point of minimum swelling of hide powder 
occurs at a pH value of 4.8, indicating this as its isoelectric point. 
Porter also found points of maximum swelling of hide powder at pH 
values of 2.4 in acid solution and about 12.3 in alkaline solution. 

Thomas and Kelly compiled a list of isoelectric points of different 
proteins, taken from the literature, and these are reproduced in Table 
XII in terms of pH value. 


Two Forms of Collagen and Gelatin. 


Quantitative experiments upon alkaline swelling are rendered diffi- 
cult by the tendency for: the gelatin to pass into solution, which is 
very much more marked than for acid swollen gelatin. That gelatin and 
some other proteins undergo a change of form in alkaline solutions 
is apparent from recent experimental data. Lloyd ** observed a rather 
significant change occurring in gelatin dissolved in alkaline solution. 
A comparison between gelatin dissolved in acid solution and gelatin 
dissolved in alkaline solution was made as follows. 

Two grams of gelatin were put into a flask containing 200 cubic cen- 
timeters of tenth-molar hydrochloric acid. After 6 days at 20° C., the 
gelatin was completely dissolved and 20 cubic centimeters of molar 
sodium hydroxide were added to the solution, which was then tested 
and found to be neutral to litmus. 220 cubic centimeters of saturated 
ammonium sulfate solution were then added and a white, flocculent 
precipitate formed, which was filtered off. The filtrate was tested and 
found to be free from protein. The precipitate was insoluble in cold 
water and was washed several times. It was dissolved in 2 cubic 
centimeters of hot water and set to a jelly upon cooling. A control 
experiment made by dissolving 2 grams of gelatin in 220 cubic centi- 
meters of water with 1.12 grams of sodium chloride behaved in a 
similar manner. 

1 W. B. Hardy. Proc. Roy. Soc. 66 (1900), 110, 


12 Biochem. Z. 41 (1912), 373. 
18 The Isoelectric Point of Collagen. A. W. Thomas and M. W. Kelly. J. Am. Chem. 
Soc. 44 (1922), 195. 
% Swelling of Hide Powder. E.C. Porter. J. Soc. Leather Trades Chem. 5 (1921), 259, 
and 6 (1922), 83. of 
. 7°On the Swelling of Gelatin in Hydrochloric Acid and Caustic Soda. D. J. Lloyd. 
Biochem, J. 14 (1920), 147. 


110 THE CHEMISTRY OF LEATHER MANUFACHGRS 


For comparison, 2 grams of gelatin were put into a flask con- 
taining 200 cubic centimeters of tenth-molar sodium hydroxide. The 
gelatin was completely dissolved after 2 days at 20°C. 20 cubic centi- 
meters of molar hydrochloric acid were then added to the solution, 
after which it reacted neutral to litmus. 220 cubic centimeters of sat- 
urated ammonium sulfate solution were added and a white, flocculent 
precipitate formed, which was filtered off. The filtrate, as in the 


TABLE SAIL 


SWELLING OF GELATIN IN PHOSPHATE BUFFER SOLUTION DurRING 4 Days at 7° C. 


pH value of buffer solution Increase in wt. of 
Sh we 1 g. dry gelatin 

Initial  - Final Grams 
2.90 2.92 13.20 
3.50 3.50 9.49 
3.96 4.01 7 pie 
4.14 4.17 6.91 
4.47 4.59 6.68 
4.78 4.86 6.20 
5.08 5.12 7.02 
5.29 5.38 7115 
5.57 5.61 ye 
5.78 5.80 7.56 
6.04 6.08 7.80 
6.209 6.29 7.83 
6.48 6.49 8.02 
6.69 6.70 8.29 
6.96 6.904 8.31 
7.08 7.10 8.25 
7-41 787 8.03 
7.68 7.62 7.62 
7.97 7.89 8.30 
42 8.36 8.59 
8.56 8.48 8.60 
9.03 8.06 8.78 
9.57 9.51 8.91 
10.00 9.96 8.98 
10.47 10.41 9.24 
11.06 10.98 9.55 
11.52 11.48 9.95 
12.00 F105 10.73 


previous experiment, was found to be free from protein. But the 
precipitate dissolved completely and rapidly in a small volume of cold 
water and would not set to a jelly even when the volume was reduced 
to 2 cubic centimeters. 

Lloyd suggested that gelatin changes from a keto-form to an enol- 
form in alkaline solution. The gelatin recovered from acid solution 
and which had the power of setting to a jelly would thus be regarded 
as the keto-form of gelatin, while that recovered from alkaline solu- 
tion and which had lost the power of setting to a jelly would be 
looked upon as the enol-form of gelatin. Miss Lloyd regarded the 
change in alkaline solution as irreversible, but her experiments do not 


Peeer AL COBRMISTRY OF THE PROTEINS It 


show this. Mr. Kern, in the author’s laboratory, added hydrochloric 
acid to gelatin dissolved in a hot solution of sodium hydroxide until 
the pH value, as determined by the hydrogen electrode, was reduced 
to 4.7 and then allowed the solution to cool, whereupon it set to a 
firm jelly, indicating that the change is reversible. Miss Lloyd’s ex- 


Increase in Weight of 1 Gram Dry Gelatin (Grams) 


4 5 6 7 8 9 10 Selle 12 
pH Value of Buffer Solution 


Fic. 45.—Showing the two points of minimum swelling of gelatin. 


periment showed merely that it is not readily reversed by the addition 
only of the quantity of hydrochloric acid equivalent to that of the 
sodium hydroxide originally employed. 

In studying the degree of plumping of calf skin as a function of 
pH value, Wilson and Gallun found two points of minimum, one at 
5.1 and the other at 7.6. This work will be described in Chapter 9. 
Wilson and Kern 7° followed this with a series of experiments upon the 
swelling of gelatin in buffer solutions and also found two points of 


minimum, one at 4.7 and the other at 7.7. A description of their 
work follows. 


%*The Two Forms of Gelatin and Their Isoelectric Points. J. A. Wilson and E. J. Kern, 
J. Am. Chem, Soc. 44 (1922), 2633. 


112 THE CHEMISTRY OF LEATHER MANUFACTURE 


A series of buffer solutions was prepared, each member of which 
had a final concentration of tenth-molar phosphoric acid plus the 
amount of sodium hydroxide required to give the desired pH value as 
determined by the hydrogen electrode at 20° C. The pH values ranged 
from 3 to 12. 200 cubic centimeters of each solution were put into a 
stoppered bottle and kept in a thermostat refrigerator at 7° C. After 
the temperature of each solution had reached 7°, a small strip of 
high grade gelatin of known weight was put into it. All strips were 
taken as nearly alike as possible and were kept in the solutions at ee 
for 4 days, after which each strip was quickly blotted off and weighed. 
The results were carefully rechecked. In Table XIII are given the 
gain in weight per gram of dry gelatin and the initial’ and final pH 
values of the buffer solutions. Fig. 45 represents the degree of 
swelling as a function of the pH value. 

Wilson and Kern suggested that the two points of minimum rep- 
resent the isoelectric points of the two forms of gelatin described by 
Lloyd and this view appears to be substantiated by other data available 
in the literature. 

Experiments upon the mutarotation of gelatin led Smith 17 to sug- 
gest that gelatin exists in two forms: a sol form, having a specific 
rotation of [a]p =— 141 and being stable at temperatures above 36° 455 
and a gel form, with a specific rotation of [a]p = — 313 and stable 
under 15°, a condition of equilibrium existing between the two forms 
at intermediate temperatures. The gel form is characterized by its 
power to set to a jelly, which is lacking in the sol form. Smith cal- 
culated that a concentration of from 0.6 to 1.0 gram of the gel form 
per 100 cubic centimeters is required to produce gelation. As the 
temperature is increased above 15°, the total concentration of gelatin 
required to produce gelation is increased because of the decreasing 
proportion of the gel form, which does not exist at all above et 
Gelatin is the only protein known to show mutarotation, but it gradually 
loses this property along with its jellying power, when its solutions 
are kept at temperatures above 70° C. 

Davis and Oakes #* measured the viscosities of a series of solutions 
of gelatin at 40° C. at different pH values. Their results are shown jn 
Fig. 46. A point of minimum occurs at 8, but none at 4.7, the iso- 
electric point of gelatin as determined by Loeb. They commented upon 
this as follows: “There may be considerable difficulty in reconciling 
this minimum viscosity at pH about 8 with the isoelectric point at 
pH 4.7.” But Davis and Oakes really measured the point of minimum 
viscosity of the sol form, since their determinations were made at 40° C., 
whereas Loeb determined the isoelectric point of the gel form. 

Another case of the apparent disappearance of an isoelectric point 
when working at a temperature of 40° C. is to be found in the work of 
Wilson and Daub,!® who experimented upon the bating of calf skin at 


7 Mutarotation of Gelatin and Its Significance in Gelation. C. R. Smith. J. Am. Chem. 
Soc. 41 (1919), 135. 

* Further Studies of the Physical Characteristics of Gelatin Solutions. C. E. Davis and 
Er: Oakes. _J. Am, Chem. Soc. 44 (1922), 464. 
: ae Critical Study of Bating. J. A. Wilson and G. Daub. J, Ind. Eng. Chem. 13 
1921), 1137. 


Pee ioa CHEMISTRY OF THE PROTEINS 113 


40° at different pH values. They observed that a point of minimum 
plumping occurred in the region of pH = 8, but not at pH = 5, the 
isoelectric point of collagen found by Thomas and Kelly and by Porter. 
But Wilson and Gallun observed points of minimum plumping of calf 
skin at both 5 and 8, when working at low temperatures. The recent 
work of Sheppard, Sweet and Benedict 2° adds further evidence of 
the existence of critical pH values at both 5 and 8. They obtained a 


1.60 
1,50 


1.40 


Absolute Viscosity 


1,30: 
1,20 
ee he, 


1,00 


DS oad: Sep ipealye cab? 
pH Value of Gelatin Solution 


Fic. 46.—Variation of viscosity of I-per cent solution of gelatin at 40° C. 
with change of pH value. 


curve for the rigidity of gelatin jelly as a function of pH value exhibit- 
ing a shoulder at 5 and a flattish maximum between 7 and 9. 
Apparently the change in gelatin from the gel form to what has 
been called the sol form takes place both with rise of temperature and 
with rise of pH value. Since the experiments of Wilson and Kern 
were performed at 7° C., they were dealing with the gel form of gelatin 


20 Elasticity of Purified Gelatin Jellies as a Function of Hydrogen-Ion Concentration, 
S, E. Sheppard, S. S. Sweet and A. J. Benedict. J. Am, Chem. Soc. 44 (1922), 1857. 


114 THE CHEMISTRY OF LEATHER MANUFACTURE 


in acid solution and actually observed a point of minimum at pH = 4.7, 
the isoelectric point of the gel form. The appearance of a second point 
of minimum swelling at pH = 7.7 seems to indicate that between 4.7 
and 7.7 the gelatin passes from the gel to the sol form and that the 
second point of minimum occurs at the isoelectric point of the sol 
form. It was only by working at temperatures as low as 7° that they 
were able to prevent the gelatin from passing into solution at the 
higher pH values. 

While objection may be raised to the terms gel and sol form as 
applied to the two forms of gelatin and of collagen, they will serve 
as well as any until more is known of the transition. Lloyd’s 
suggestion that the change is a keto-enol tautomerism is. still 
speculative. 

Parker Higley,*! at the University of Wisconsin, has recently in- 
vestigated the absorption spectra of gelatin dispersions of different pH 
value and plotted a series of curves, at several densities, for the wave 
length of maximum absorption in the ultra violet as a function of pH 
value. The curves all show two points of minimum, one at pH = 4.68 
and the other at 7.66, coinciding with the points of minimum swelling 
_ of gelatin. That the two points of minimum have a real existence 
is thus strikingly confirmed from an unexpected source. 

The effect upon vegetable tanning of the change of one form of 
collagen to the other will be discussed in Chapter 13. 


Electrical Potential Difference between Protein Jelly and Aqueous 
Solution. 


It is apparent from the discussion of Donnan’s theory of membrane 
equilibria that the unequal distribution of ions between a jelly and 
its surrounding solution must give rise to an electrical difference of po- 
tential between these two phases whose measure is (RT/F) .log(x/y), 
where x is the hydrogen-ion concentration of the external solution and 
y that of the solution within the jelly and this value holds true re- 
gardless of the valence or number of ions in the system. The potential 
difference can therefore be calculated from the determinations of pH 
value in the jelly and in the external solution. Changing from natural 
to common logarithms and substituting the numerical value for RT/F 
at 20° C., we get. 


P.D. = 58log(x/y) = 58(logx — logy) millivolts. 


But —log y= pH value of the jelly and + log 4 = —pH value of 
the solution. Hence, BL 20 21, 


P.D. = 58(pH of jelly minus pH of solution) millivolts. 


Loeb ** devised a very ingenious method for determining this potential 
difference directly by means of a pair of calomel cells of equal value 


21 Advance note. 
#2 Cf, Proteins and the Theory of Colloidal Behavior, Pp. 154, 


PRYSiCAL CHEMISTRY OF THE PROTEINS 115 


and a Compton electrometer. A diagram of his apparatus is shown 
in Fig. 47. The potential difference measured is that of the cell 


calomel 
electrode 


saturated 


KCl 


solid 
jelly 


external 
solution 


saturated 
roel 


calomel 
electrode 


Everything else being symmetrical, the potential difference measured 
is that between the jelly and the external solution with which it is 
supposed to be at equilibrium. 

In a typical experiment,?* 1 gram of purified gelatin, powdered to 
a grain size between 30 and 60 mesh, was put into each of a series of 
solutions of different concentrations of hydrochloric acid or sodium 


Fic. 47.—Loeb’s apparatus for measuring the potential difference between gela- 
tin jelly and the surrounding solution. 


hydroxide. The volume of each solution was 350 cubic centimeters and 
the temperature 20° C. After 4 hours the volume occupied by each 
portion of gelatin was measured, the solution filtered off, and the 
gelatin melted so that the pH values of both jelly and solution could 
be determined by means of the hydrogen electrode. The gelatin was 
then allowed to set to a jelly in the receptacle illustrated in Fig. 47 
and the potential difference between the jelly and external solution 
was then measured with a Compton electrometer. The results of 
such a series are shown in Table XIV along with calculations of the 
potential differences made from the pH determinations. The cal- 
culated and observed results are at least of the same sign and order 
of magnitude, which is a good agreement considering the nature of 
the experiments and the dilutions of the solutions. It will be shown 
later that the method is capable of very much better agreement where 
the complications involved in melting and resetting of the jelly are 
avoided, as in the measurement of potential difference between a 
solution of gelatin and a protein-free solution with which it is in 
equilibrium and from which it is separated by a semi-permeable 
membrane, especially where the solutions have greater conductivities. 


78The Origin of the Electrical Charges of Colloidal Particles and of Living Tissues. 
Jacques Loeb. J. General Physiol. 4 (1922), 351. 


116 THE CHEMISTRY OF LEATHER MANUFACTURE 


According to the theory, the concentration of free acid in an acid- 
swollen jelly should be less than that in the external solution and, 
likewise, the concentration of free alkali in an alkali-swollen jelly should 
be less than that in the external solution with which it is in equilibrium. 


TABLE XIV. 
SUSPENSIONS OF POWDERED GELATIN. 
After 4 hours at 20° C. 


pH value of : 
Vol. of Absorbed External (a) P.D. millivolts 


Initial normality gelatin solution solution minus  Calcu- 
of solution (mm.) (a) (b) (b) lated Observed 

O.0OLONGE Cl -- wues ees 28 4.44 235 + 1.09 + 63.0 + 56.0 
D.00OSN PE Clas nates 20 4.56 3.55 + 1.01 + 58.6 Ache 
O.0000N EiGl ewes eae 18 4.79 3.0245 Oe + 51.0 + 26.5 
O:000IN SHES. a eae 16 4.85 4.24 + 0.61 + 36.0 + 15.0 
Wekett uaGctnet ese coleanas 17 4.89 4.97 — 0.08 — 4.5 — 17.5 
O.000IN- Na@H Wor... 18 4.98 5.96 — 0.98 — 57.0 — 59.0 
0,.0002N"= NaOH... ...ci5 9 28 5.06 6.24 —1.18 —680 — 61.0 
0.0005N NaOH 7.4 50.2.4. 537 5.50 6.46 — 0.96 — 56.0 — 70.0 
COOION  NaOHus.. 3. ss d0 6.74 7.30 — 0.56 — 33.0 — 66.0 
0.0020N NaOH) 2225... .. 47 0.54 10.56 —1.02 —59.0 —46.0 
0.0040N NaOH ......... 48 10.15 11.08 — 0.93 — 48.0 — 36.0 


This is verified by the figures in Table XIV, which show, for pH 
values of the external solution less than 4.7, that the hydrogen-ion 
concentration is greater in the solution than in the jelly, while for pH 
values of the external solution greater than 4.7, the hydrogen-ion con- 
centration is less or the hydroxide-ion concentration greater in the 
solution than in the jelly. 


Rhythmic Swelling of Protein Jellies. 


Sheppard and Elliott 24 made a study of the causes of the reticula- 
tion of the surfaces of photographic negatives that has a bearing upon 
a similar kind of trouble sometimes occurring in the vegetable tanning 
of skins. During the fixing or washing of a negative, the wet gelatin 
layer sometimes becomes more or less finely wrinkled or corrugated, 
the network of puckers forming a pattern extending either over the 
whole of the negative or only over part of it. They found that this 
reticulation can be produced by the combined action of a swelling 
agent and a tanning agent. 

Fig. 48 represents a print from a negative treated to produce reticu- 
lation by Mr. Daub in the author’s laboratories. The plate was flashed, — 
developed, fixed with sodium thiosulfate, washed, and then immersed 
in a solution of wattle bark extract containing 5 grams of tannin 
and 0.2 mole of acetic acid per liter; the temperature was kept at 


24 The Reticulation of Gelatine. 


S. E. Sheppard and F. A. Elliott. J. Ind. Eng. Chem. 
10 (1918), 727. ri 


PHYSICAL CHEMISTRY OF THE PROTEINS 117 


28°C. After several minutes the gelatin surface began to pucker at 
isolated points and this action gradually spread over the entire sur- 
face, producing series of ridges of swollen gelatin with valleys of 
hardened and contracted gelatin in between. Following this action, 
the silver particles migrated from the hardening portions into the 
swelling ridges, giving the negative the mosaic-like appearance shown 
in the print. Often the puckering became well pronounced before the 


Fig. 48.—Reticulation Produced on Photographic Negative. 


migration of silver particles was noticeable. Sheppard and Elhiott 
liken the effect to the production of Liesegang rings. 

The acid tends to cause a swelling of the gelatin while the tannin 
tends to cause a hardening and contracting action. But the acid diffuses 
relatively very rapidly whereas the diffusion of the tannin is greatly 
retarded both by its high molecular weight and by its tendency to 
combine with the gelatin, forming a compound less permeable and 
having a much lower power of swelling than the original gelatin. The 
action becomes greatly accelerated as the temperature is raised towards 
the melting point of the gelatin jelly. When the action is prolonged 


118 THE CHEMISTRY OF LEATHER MANUFACTURE 


at higher temperatures, provided the jelly does not dissolve, a second 
and much coarser series of puckers begins to form, tending to mask 
the finer pattern. In the coarser pattern, the peaks of the ridges may 
be from one to several millimeters apart. 

The reticulation of the surface of skin in tanning is a very serious 
matter as the pattern formed is permanent and materially reduces the 
selling value of the leather. The pattern formed on skins is usually 
of the coarser variety and would hardly pass as an artistic sample of 
embossing, which the photographic negative might do, because of the 
fineness of the pattern and distribution of silver particles. The reticu- 
lation of skin may attend the injudicious use of acid in attempting to 
plump the leather during tanning, or it may occur where acid-producing 
ferments get the upper hand in a yard where fresh liquors are normally 
used. The corrective is to prevent the swelling action, either by 
neutralization of the acid or by the addition of salt. 


Structure of Gelatin Solutions and Jellies. 


Procter’s *° investigations of the behavior of gelatin jellies led 
him to regard them as having a structure consisting of a network of 
molecules cohering to each other, but leaving interstices large enough 
to permit the passage of water and simple molecules and ions. The 
long chains of amino acids making up the protein molecules are 
peculiarly fitted to produce such a structure through combination of the 
acid and basic terminals of these chains. A hot solution of gelatin 
may be looked upon as a true solution consisting of individual gelatin 
molecules, or at least of comparatively small polymerized groups, but 
the molecules orientate themselves, as the solution cools, so as to leave 
a minimum of free energy, the most active acid groups tending to 
unite with the most active basic groups until a continuous network 
is formed throughout the system. A block of jelly might thus be 
looked upon as an enormous, single molecule. Such a view is not 
radical in the light of modern theories of crystal structure. 

According to the Procter-Wilson theory of swelling, when a block 
of gelatin jelly is immersed in a solution of hydrochloric acid, the 
solution passes into the jelly, filling up the interstices. Of the ionized 
gelatin chloride, which then forms, the chloride ions remain in the 
solution in the interstices while their corresponding gelatin cations 
form part of the network and are not in solution in the same sense 
as the anions. In tending to diffuse into the outer solution, the anions 
exert a pull upon the cations forming part of the network, causing 
an increase in volume of the jelly proportional to the pull exerted, 
so long as the elastic limit is not exceeded. That gelatin jellies are 
truly elastic and follow Hooke’s law may be taken as proved chemically 
by the agreement between calculated and observed results shown in 
Table XI. More recently Sheppard and Sweet 28 proved by measure- 

7° The Structure of Organic Jellies. H. R. Procter. Proc, Seventh International Con- 
gress of Applied Chemistry, London, 1909. 


*° The Elastic Properties of Gelatin Jellies. S, E. Sheppard and S. S. Sweet. J. Am. 
Chem. Soc. 43 (1921), 539. } 


PreAl: CHEMISTRY OF THE PROTEINS 119 


ments of rigidity that gelatin jellies follow Hooke’s law nearly up 
to the breaking point. 

Loeb’s work on the viscosity of gelatin solutions, to be discussed 
presently, indicates that the initial step in gelation is the combination 
of individual molecules to form large aggregates, possibly in a manner 
similar to the growth of crystals. Bogue *’ pictures this process as 
the formation of catenary threads by the union of the individual 
molecules end to end. The manner in which fibrous curds of soap 
are formed led McBain ?® to a similar view regarding the structure of 
soap jellies and solutions. He attributes the elasticity of gels to the 
formation of an exceedingly fine filamentous structure. Innumerable 
molecules placed lengthwise and held together by forces of residual 
valence are assumed to make up these fine threads, which may be 
microns or millimeters in length. 

Considering the nature and variety of the amino acids composing 
the gelatin molecule, as shown in Table I of Chapter 3, we should 
hardly expect the polymerization of gelatin to take place along a 
single line, but in every direction and probably with cross chains grow- 
ing to support chains increasing in length in other directions. The 
increasing viscosity of gelatin solutions with time, upon cooling, would 
thus be attributed to the increasing size of the particles; the formation 
of a rigid jelly to the final union of the large particles, forming a 
structure continuous throughout the entire system. 

There is an abundance of evidence to support Procter’s view of 
the structure of jellies and Loeb’s view that gelatin solutions, after 
standing for a time at temperatures below 35° C., always contain par- 
ticles of jelly consisting of aggregates of gelatin molecules. A number 
of supporting lines of evidence are given in a review of the literature 
by Thompson.?® 

Graham showed long ago that the velocity with which crystalloids 
diffuse through gelatin jellies is only very little less than the velocity 
through pure water. This slight reduction in velocity is in no way 
comparable with the apparently great physical difference in state be- 
tween the jelly and water. Although the viscosity of a gelatin jelly 
is too great to be measured by the methods usually applied to liquids, 
simple molecules move through it as though in a medium of viscosity 
nearly that of water. The network theory explains this by assuming 
that the diffusing substance actually is moving through the pure water 
or aqueous solution in the interstices of the network. Any slight 
diminution in velocity can be accounted for by the small portion of 
any cross section of the jelly occupied by the gelatin network. The 
same holds true for gelatin solutions, the diffusing substance being able 
to pass through the particles of jelly in suspension almost as rapidly 
as through the solution surrounding the particles. 


ts ga and Constitution of Glues and Gelatines. R. H. Bogue. Chem. Met. Eng. 
23 (1920), 61. 

78 Colloid Chemistry of Soap. J. W. McBain. Brit. Assoc. Advancement Sci. Third 
Report on Colloid Chemistry (1920), 2. 
( pies of Gelatin Solutions. F. C. Thompson. J. Soc. Leather Trades Chem. 3 
T1919), 209. 


120 THE CHEMISTRY OF LEATHER MANUFACTURE 


Thompson shows from the work of Dumanski *° that the con- 
ductivity of a solution of potassium chloride in gelatin jelly is no less 
than in pure water when a correction is made for the small volume 
actually occupied by the gelatin network, whereas, if the apparent 
viscosity had any effect, the conductivity should be reduced by the 
gelatin to\a minute fraction of its value in pure water. | 

The vapor pressure of even a 20-per cent gelatin jelly is prac- 
tically the same as that of water, indicating the presence of pure 
water in accordance with the network theory. 

By placing a strain upon gelatin jelly in one direction, double re- 
fraction is produced, a property always associated with a definite 
structure and with anisotropy. Even dilute solutions of gelatin show 
double refraction on compression or when passed between two cylinders 
rotating in opposite directions. With increasing strain, the effect is 
increased up to a point corresponding to an elastic limit. This in- 
dication of structure even in gelatin solutions corroborates the views of 
Loeb and of Bogue. 

The fact that the viscosity of gelatin solutions is lowered by simply 
agitating the solution is another piece of evidence in favor of the 
existence of a structure in gelatin solutions and still further evidence 
is furnished by Loeb’s work on the viscosity of gelatin solutions and 
Bogue’s measurements of plasticity, to be described later. 


Relation of the Osmotic Pressure and Viscosity of Gelatin Solu- 
tions to the Swelling of Gelatin Jellies. 


In an extensive series of experiments, Loeb has shown that the 
variations in osmotic pressure and viscosity of gelatin solutions with 
change of pH value or of concentration of salt, parallel the corre- 
sponding variations in the degree of swelling of gelatin jellies, which 
is what would be expected on the basis of the theory of protein-salt 
formation described above. This parallelism is shown by the curves 
in Figs. 49 to 54. 

In each determination *! of the two series of experiments performed 
to get the curves shown in Fig. 49, 1 gram of powdered gelatin 
was put for 1 hour at 20° C. into 100 cubic centimeters of acid solu- 
tion of definite strength. The volume of the gelatin was measured, 
after settling, in a graduated cylinder and the pH value of the jelly 
was determined after melting. The volume is plotted against the pH 
value of the jelly and not that of the external solution, which was 
always lower, as explained in the discussion of the theory of swelling. 

The curves in Fig. 50 were obtained by rapidly heating to 45° C. 
solutions of 0.8-per cent gelatin containing different amounts of acid, 
maintaining this temperature for 1 minute, cooling rapidly to 24°, 
and immediately determining the viscosity at 24°. The viscosity is 
plotted against the pH value of the gelatin solution.®2 


°° Z. physik, Chem. 50 (1907), 553. 

*1Ion Series and the Physical Chemistry of the Proteins; II. Jacques Loeb. J. General 
Physiol. 3 (1920), 247. 

Ton Series and the Physical Properties of Proteins; I. Jacques Loeb. J. General 
Physiol, 3 (1920), 85. om 


pee sICAL CHEMISTRY OF THE PROTEINS I2I 


In the experiments whose results are shown in Fig. 51, collodion 
bags, cast in the form of Erlenmeyer flasks having a volume of 50 
cubic centimeters, were filled with I-per cent gelatin solutions con- 
taining different amounts of acid. Each bag was closed with a rubber 
stopper fitted with a glass tube serving as a manometer and put into 


4 
he Temp. 20°. 
fs 60 
ae 40 
pr HeS04 
Oe 
» 
Ba 20 
® 
o 
© 1 gram of powdered gelatin 
Pe 


0.8 gram gelatin dissolved in 
LOO-c. c,. solution 


Relative Viscosit 
(water = 1) 


4007 4 gram gelatin 
dissolved in 
LOO) G, uC. 
solution 


HoS04 


Temp. 24°. 


Osmotic Pressure 
(m. m. of water) 
re) 
ro) 
oO 


2 3 4A 
pH Value of Gelatin Solution 


Variables as Functions of pH Value. 


Fic. 49.—Volume of powdered gelatin. 
Fic. 50.—Viscosity of gelatin solution. 
Fic. 51—Osmotic pressure of gelatin solution. 


a beaker containing dilute acid solution of the same kind as was used 
in making up the gelatin solution. When osmotic equilibrium was 
established, the level of solution in the manometer was recorded 
and plotted against the pH value of the gelatin solution.** The 
measurements were made at 24°. 

The explanation of the parallelism between the curves for swell- 
ing, viscosity, and osmotic pressure as functions of pH value is that 


88 Donnan Equilibrium and the Physical Properties of Proteins; II, Osmotic Pressure. 
Jacques Loeb. J. General Physiol. 3 (1921), 691. 


122 THE CHEMISTRY OF LEATHER MANUFACTURE 


the variation in each case is due to the same fundamental cause, namely, 
the establishment of a Donnan equilibrium. In the viscosity measure- 
ments, the solutions contain aggregates of gelatin molecules capable 
of swelling with change of pH value and, since the viscosity must in- 
crease with the increasing volume occupied by the gelatin, we should 


3 25 
oo 
a 0.5 gram of powdered gelatin 
pa ie 8 
uA Temp.<20°, 
on 16 pH value 3.0. 
fo 
do 
od 10 
hb 
i 0.5 gram of powdered gelatin 
o 2.5 suspended in 100 c. c. of 
on solution 
=" 2.0 | 
ake : Temp. 20°. 
PP pH value 3.0. 
re 
BE 1.5 
rs 
i?) 
cr 
400 

o~ 1 gram of gelatin dissolved 
26 in 100 c. c. of solution 
@ ® 300 
ope Temp. 24°, 

u pH value 3.5. 
2 200 
at s 
> #8 
e 
a #100 


0,02 0.04 0.06 
Moles NaCl or NaNOg per Liter 


Variables as Functions of Concentration of Added Salt. 


Fic. 52.—Volume of powdered gelatin. 
Fic. 53.—Viscosity of gelatin suspension. ' 
Fic. 54.—Osmotic pressure of gelatin solution. 


expect the viscosity to rise and fall with the degree of swelling of the 
gelatin particles. 

In the experiments on osmotic pressure, we have an application of 
the Donnan equilibrium which is considerably simpler than that involved 
in the swelling of jellies, although of a similar kind. 

In the swelling and osmotic pressure experiments, we note that the 
points of maximum given by sulfuric acid are only half as great as 
those given by hydrochloric acid, which is in harmony with the theory, 
since the divalent sulfate ion has no greater diffusion pressure than 


PHYSICAL CHEMISTRY OF THE PROTEINS 123 


the monovalent chloride ion and is only half as numerous for equivalent 
concentrations of gelatin salt. 

In Figs. 52, 53, and 54 are given curves showing the depressing 
effect of increasing concentration of neutral salt upon the volume of 
powdered gelatin,** the viscosity of a suspension of powdered gelatin,** 
and the osmotic pressure of a solution of gelatin.*® Again we find a 
parallelism in the results that would be expected from the theory. 


Osmotic Pressure and Membrane Potentials. 


A discussion of the mechanism of the osmotic pressures exerted 
by protein solutions may serve to make the theory of swelling, which 
is the more important in leather chemistry, a little clearer. The 
collodion bags used in Loeb’s experiments were permeable to water and 
simple acids, bases, and salts, but not to dissolved proteins. Let us 
consider a solution of gelatin chloride and hydrochloric acid contained 
in a collodion bag which is brought into contact with pure water. 
Hydrochloric acid diffuses out through the membrane until equilibrium 
is established between the external solution and the gelatin solution 
inside the bag. The outside solution contains only hydrochloric acid, 
but the inside solution contains both hydrochloric acid and_ gelatin 
chloride. At equilibrium, in the outside solution, let 


x = [H*] = [CV] 


and in the inside solution let 


y=([H*] | 
z = [gelatin ion] 
whence PCr y+ z. 
It is apparent from the reasoning given early in this chapter that at 
equilibrium 
| x? = y(y + 2) 
and that Oy a2 > 2X: 


The greater concentration of diffusible ions of the inside solution, 
2y-+z2, must give rise to an osmotic pressure proportional to the 
quantity ¢ in the expression 


e= 2y + z— 2x. 


This assumes that the gelatin exerts no osmotic pressure of its own, 
which may not be strictly true. A correction would have to be made 
by adding to e an amount corresponding to the osmotic pressure of the 
gelatin. But Loeb ** has shown that any such correction that may be 
necessary is less than the probable experimental error of measurement. 

* Donnan Equilibrium and the Physical Properties of Proteins; III, Viscosity. Jacques 
Loeb. J. General Physiology 3 (1921), 827. 

35 Donnan Equilibrium and the Physical Properties of Proteins; I, Membrane Potentials. 
Jacques Loeb. J. General Physiol. 3 (1921), 667. 


eo The DD Ses of the Influence of Acid on the Osmotic Pressure of Protein Solu- 
tions. Jacques Loeb. J. Am, Chem. Soc, 44 (1922), 1930. 


124 THE CHEMISTRY OF LEATHER MANUFACTURE 


When +, y, and zg are determined in the solutions, the osmotic pres- 
sure can be calculated. At 24°C. the osmotic pressure, in terms of 
millimeters pressure of a column of water, equals 2.5¢ X 10°. For 
casein chloride, Loeb found that the observed osmotic pressure ap- 
proximated the value 250000e as closely as the determinations could 
be made. 

Because of the unequal distribution of ions between the inside 
and outside solutions, there must be an electrical difference of potential 
set up between the two solutions whose measure at 20° C., as in the 
case of the jellies, is given by the formula 


P.D. = 58(pH inside minus pH outside) millivolts. 


In determining the potential difference between the inside and out- 
side solutions, Loeb used an apparatus similar to that shown in Fig. 47. 
The collodion bag containing the inside solution was hung in the. 
beaker filled with the external solution. The manometer tube of the 
collodion bag was replaced by a funnel. ‘The capillary tube of the 
right hand calomel cell was dipped into the funnel so as to make 
contact with the inside solution. ‘Lhe potential difference of the system 
was then measured by means of a Compton electrometer. S 


TABLE XV. 
GELATIN SOLUTIONS AT 24° C. 
pH value of 
Osmotic Inside Outside (a) P.D (millivolts ) 
Moles pressure solution solution minus Calcu- Ob- 
NaNO; per liter (mm. ) (a) (b) Ch lated = served 
INOUE Tt or eee 435 3.58 3.05 0.53 31.2 31 | 
O.0002d1 tera sine ee 405 3.56 3.08 0.48 28.3 28 ) 
CL0004BS On se ki cael any 3.51 3.10 0.41 24.0 24 : 
G.00007 5. oe suse Siew 335 3.46 3.11 0.35 20.7 22 i 
G.OCLQS oo ecgis none ae 280 3.41 3.14 0.27 16.0 16 ' 
COD 30 cas ki oats eee 215 3.30 ait7 0.19 11.2 12 
ODO070 cae. orca aa ec 134 3.32 3.20 0.12 7.0 7 
GORE s 2. Oe aoe 85 3.20 3:22 0.07 4.1 4 
OO3T2 cance ces ae 63 3.25 3.24 0.01 0.6 O 


Further quantitative proof of the correctness of the theory is fur- 
nished by the data in Table XV, showing the depressing effect of 
increasing concentration of neutral salt upon the osmotic pressure 
and potential difference of a system in which an acid solution of 
gelatin is separated from a gelatin-free solution by means of a collodion 
membrane.*’ The osmotic pressure curve is plotted in Fig. 54. When 
equilibrium was established, the pH values of both inside and outside 
solutions were determined and the potential differences were determined 
in the manner described above. The potential differences were also 
calculated from the pH determinations, the factor 58.8 being used for 
24°. The agreement between calculated and observed results is as 
nearly perfect as could be hoped for. 


** The Colloidal Behavior of Proteins. Jacques Loeb. J. General Physiol. 3 (1921), 557.. 
7 


PHYSICAL CHEMISTRY OF THE PROTEINS 125 


With increasing concentration of salt, the pH values of the inside 
and outside solutions approach each other. According to the theory, 
the distribution of any ion between the two solutions is similarly affected 
by the addition of salt; i.c., the logarithms of its concentration in the 
inside and outside solutions, respectively, approach each other, bring- 
ing about a lessening of the difference in total concentration of dif- 
fusible ions between the two solutions. It is this effect rather than any 
supposed repression of ionization of the protein salt that is responsible 
for the reduction in the swelling of jellies and the osmotic pressure, 
viscosity, and potential difference of protein systems. 


Changes in Viscosity of Gelatin Solutions with Time. 


When hot solutions of gelatin are allowed to cool, their viscosities 
increase with time until they finally set to rigid jellies. Loeb attributes 
this to the formation of aggregates of gelatin molecules, the viscosity 
increasing with the average size of the gelatin particles. The curves 
in Figs. 55 and 56 show that this increase in viscosity with time 


fo) 
3 
. é 
» 
& b pH value = 2.7. 
b> sc 
- } 
n oO 
° 2 
a > 
ot 
as 
Temp. 20°. 
10 20 30 40 = 50 10 20 30 40 £50 
Time in Minutes Time in Minutes 
Fic. 55.—Increase in viscosity with Fic. 56.—Change in viscosity with 
time of 2-per cent solutions of time of 2-per cent solutions of 
gelatin sulfate of different pH gelatin chloride at different tem- 
values. ; peratures. 


is materially influenced both by the pH value and temperature of the 
gelatin solution.** The effect of pH value was determined by rapidly 
heating 2-per cent gelatin solutions containing different amounts of 
sulfuric acid to 45° C., cooling rapidly to 20°, and then maintaining 


% The Reciprocal Relation_between the Osmotic Pressure and the Viscosity of Gelatin 
Solutions. . Jacques Loeb. J. General Physiol. 4 (1921), 97. ; 


120 THE CHEMISTRY OF LEATHER MANUFACTURE 


this temperature while viscosity measurements were made at intervals 
of 5 or 10 minutes. An increasing concentration of acid tends to pre- 
vent the formation of aggregates; the viscosity increases most rapidly 
at the isoelectric point. 

The effect of temperature was determined by rapidly heating 2-per 
cent gelatin chloride solutions having a pH value of 2.7, to 45°"G5 
cooling rapidly to the temperature at which the viscosity measurements 
were to be made, and maintaining this temperature while determina- 


° 


Q 
am Nrero ° 
&% D OSS S 


160 
140 
120- 
100 
; 
604! 1 
‘ / 
H i 
404!) 


pay 


Viscosity (angular deflection) 
ee) 
oO 
£06 


wire #30 
j y 
2041 Hi 

/ M,/, 


da 
4 


LO: 20 30° 40a 
Revolutions per Minute 


Fic. 57.—Viscosity-plasticity curves for a 20-per cent gelatin solution. 


tions were made at intervals of 5 or 10 minutes. The remarkable point 
to be observed is that the viscosity increases with time at temperatures 
below 35° C., but decreases with time at higher temperatures. 

Bogue *® measured the viscosities of gelatin solutions at different 
temperatures by means of a Macmichael torsional viscosimeter. At 
each temperature he made measurements for a number of different 
speeds of rotation of the cup. A set of these is shown in Pigess%, 


°° The Sol-Gel Equilibrium in Protein Systems. R. H. Bogue. J. Am. Chem. Soc. 44 
(1922), 1313. RiRaE age ok ape: nie See i ff y EE: +4 


PewiCAL GOEPMISTRY OF THE PROTEINS Te 


The continuous lines cover the range of actual observation and the 
dotted portions represent the curves extrapolated to zero speed of 
rotation. For all temperatures above 34° C. the extrapolated curves 
pass through the origin, indicating truly viscous flow. But for lower 
temperatures, the curves do not pass through the origin; they indicate 
a finite deflection for an infinitesimal speed of rotation, showing that 
here we have an example of plastic flow. The gelatin solutions at ‘lower 
temperature actually possess a measurable degree of rigidity. This is 
further evidence in support of Smith’s view that at temperatures above 
35° gelatin in solution exists in a form having no power of gelation. 
As the temperature is lowered, some of this sol form changes into a 
gel form which has the power of gelation. As the temperature is 
lowered, the proportion of gel form to sol form increases until at 15° 
and lower temperatures all of the gelatin exists in the gel form. The 
structure of the aggregates of molecules of the gel form is such as to 
impart to the solution the rigidity observed by Bogue. 

When an acid solution of gelatin contained in a collodion bag at 
20° C. is brought into equilibrium with a pure aqueous solution of 
the acid, the solution actually is separated into 3 phases. The gelatin 
solution within the bag has a hydrogen-ion concentration less than that 
of the external solution, but greater than that of the solution absorbed 
by the aggregates of gelatin molecules suspended in the gelatin solu- 
_tion. Loeb *” has shown that, with increasing proportion of aggregates 
to dissolved gelatin, the variation of pH value produces an increasing 
effect upon viscosity, but a decreasing effect upon osmotic pressure 
measurements, as would be expected. 


Theory of Salting Out and the Stability of Colloidal Dispersions. 


Protein solutions and other so-called emulsoid colloids differ from 
the suspensoid colloids, such as colloidal gold, in requiring relatively 
very high concentrations of salt to precipitate their sols. It is gen- 
erally admitted that the stability of colloidal dispersions is increased 
by the electrical charge usually associated with the particles. However, 
but little quantitative work has been done on the actual determination 
of this charge. 

Powis *°.measured the potential difference at the oil-water boundary 
of an emulsion of cylinder oil and found that the emulsion was stable 
only when the absolute value of the potential difference exceeded 30 
~ millivolts. When it was reduced to any value lying between plus or 
minus 30 millivolts, coagulation took place, but at a rate which was 
independent of the voltage. 

It was pointed out by the author *4 in 1916 that Donnan’s theory 
of membrane potentials is applicable to suspensoids as well as to pro- 
tein jellies. A gold sol may be taken as a typical example. When gold 
is dispersed in water, the presence of chloride, bromide, iodide, or 

40 The Relation between the Stability of an Oil Emulsion and the Potential Difference 
at the Oil-Water Surface eae and the Coagulation of Colloidal Suspensions. F, Powis. 


Z. physik. Chem. 89 (1914) 
= pease of Colloids. j. a "Wilson. J, Am, Chem, Soc. 38 (1916), 1982. 


128 THE CHEMISTRY OF LEATHER MANUFACTURE 


hydroxide ion in concentrations ranging from 0.00005 to 0.005 normal 
has a marked stabilizing effect on the sol produced and the particles 
are negatively charged. The effect seems to be due to the ability of 
these ions to form stable compounds with the gold. Fluoride, nitrate, 
sulfate, and chlorate ions decrease the stability of gold sols, which 
is significant in view of the fact that they do not form stable compounds 
with gold.*? 

In Fig. 58 let A and B represent two gold particles stabilized by 
potassium chloride. In combining with the gold, the chloride ions have 
imparted their negative charges to the particles. But the potassium 
ions are still left in solution, although their field of motion is restricted 
to the thin film of solution wetting the particles because they must 
continue to balance the negative charges on the particles. The volume 


P'tG. 58.—Particles of stable gold sol showing enveloping films of aqueous 
solution. 


of the film of aqueous solution enveloping a particle will be measured 
by the surface area of the particle and the average distance that the 
potassium ions are able to travel from the surface. 

Let us now consider the case where an amount of potassium chloride — 
is present in the sol too small to cause precipitation. The enveloping 
film will contain potassium ions balancing the charges on the par- 
ticles as well as ionized potassium chloride. The surrounding solution 
will have potassium and chloride ions only in equal numbers. In the 
surrounding solution let 


x = [K*] = [CV] 


in the enveloping film let 


y= [Cl] 
and z= [K*] balanced by charges on the particles, 
whence y +z represents the total concentration of potassium ion. 


As was shown in the discussion of Donnan’s theory, the product 
[K*]  [Cl’] must have the same value both in the enveloping film and 
in the surrounding solution at equilibrium. Hence 


x? = y(y +z). 


“ The Electrical Synthesis of Colloids. H. T, Beans and H. E. Eastlack. J. Am. Chem, 
S06. 37 (1915), 2667. 


PHYSICAL CHEMISTRY OF THE PROTEINS 129 


The surface layer of solution will have a greater concentration of ions 
than the surrounding solution by the amount 2y + z— 2x. This un- 
equal distribution of ions will give rise to a difference of potential 
between the enveloping film and the surrounding solution whose 


measure is 


RE Scout 2x 
E = —— log — = —=— log ———————__— 
FP ay im —z+ V74x*+2 


But now, if we increase + without limit while g remains constant, E 
must decrease, approaching zero as a limit, since 


limit 
{ieee a log ee 


xX — © B 


—= «= OA. 
V Ax? 


It is thus evident that the difference of potential between the envelop- 
ing film and the surrounding solution will be a maximium when there 
is no free potassium chloride present and will decrease, approaching 
_ zero, as the concentration of potassium chloride is increased without 
limit. 

The particles shown in Fig. 58 are prevented from coalescing be- 
cause there is a sufficiently high potential difference of the same sign 


Fic. 59.—Coagulation of gold sol initiated by reduction of potential difference 
between enveloping films and the surrounding solution, by the addition of 


potassium chloride. 


between the surrounding solution and each enveloping film. The 
electrostatic repulsion is determined by this potential difference rather 
than by the absolute electrical charge on the particles because the surface 
film completely envelops the particles and endows them with its own 
properties. | 

When enough potassium chloride has been added to lower the 
potential difference to a point where it is no longer able to overcome 
the attractive forces between the particles and the surface tension of 
the enveloping film, the particles move toward each other and the 
enveloping films of two or more particles blend into one, as shown in 
Fig. 59. It is at this point that the actual charges themselves come into 
play and probably determine the nature of the precipitate. 


130 fo CHEMIST eee ea MANUFACTURE 


We have now only to substitute for the solid particle with its 
enveloping film the molecular network with aqueous solution filling up 
the interstices to make this theory of salting out apply to gelatin and 
similar proteins. 

By referring back to Loeb’s data in Table XV, it will be noted that 
the potential difference between an acid solution of gelatin and a gelatin- 
free solution with which it was in equilibrium was reduced to less 
than one millivolt by the addition of 0.031 mole per liter of sodium 
nitrate. If we may assume a similar lowering of potential difference 
between highly dispersed gelatin particles and the dispersion medium 
by the addition of this quantity of salt, it would follow that coagula- 
tion as a function of this difference of potential is not independent 
of the properties of the disperse phase. A gelatin solution shows no 
tendency to precipitate in the presence of 0.03 mole of sodium nitrate, 
but Powis found that his emulsion of cylinder oil ceased to be stable 
when the potential difference was reduced to 30 millivolts. Half- 
saturating a gelatin solution near the neutral point with ammonium 
sulfate will cause its precipitation, but we have as yet no data in- 
dicating the extent to which the potential difference is lowered before 
the precipitation begins. 

Thomas * has called attention to the fact that the stability of col- 
loidal dispersions may be determined more, in some cases, by the at- 
traction between the dispersed phase and dispersion medium than by 
the difference of potential at the interface. The low degree of attrac- 
tion between oil and water was probably responsible for the coagula- 
tion of Powis’ emulsion at 30 millivolts. Apparently a potential 
difference of less than one millivolt is sufficient to prevent the precipita- 
tion of certain protein solutions because of the attraction existing be- 
tween the protein and water. The attraction between sugar and water 
appears to be so great that no potential difference at all is required 
to keep it‘in solution. 

Loeb’s work, taken in conjunction with investigations in the author’s 
laboratories, indicates that the lowering of the potential difference of 
protein systems is not brought about by repression of ionization of 
the protein salts, as has often been supposed, but rather by the mecha- 
nism of the Donnan equilibrium just described. In gelatin systems in 
which the potential difference has been lowered to a very small value, 
we find no repression of ionization of gelatin chloride measurable by 
means of calomel electrodes. Moreover, there is no need to postulate 
such repression in order to account quantitatively for the observed 
results. | 

An application of this theory of salting out to soap solutions 
furnishes a needed addition to McBain’s** theory of soap solu- 
tions, in .which it would be well also to look upon the micelle 
as an aggregate of monovalent ions rather than as a complex polyval- 
ent 10n. 
aiwecee gelation Theory of (Collada! Diveersion, 6 a 


** Colloidal Electrolytes. Soap Solutions and Their Constitution. J. W. McBain and 
C. S. Salmon. J. Am, Chem, Soc. 42 (1920), 426. ‘ a 


a a a 


PHYSICAL CHEMISTRY OF THE PROTEINS 131 


Adsorption. 


Ever since Gibbs showed that the concentration of the solute must 
be greater at the surface than in-the bulk of solution where the solute 
lowers the surface tension of the’ solution, there has been a tendency 
to look upon this proof as an explanation of the fact that substances 
of great specific surface reduce the concentration of solute in many 
different kinds of solution with which they are brought into contact, 
The error in this tendency lies in the fact that Gibbs’ work applies 
only to the lowering of the surface tension by a substance actually 
in solution. Since, in many cases, it has not been found possible to 
determine the actual concentration of solute in the layer of solution 
immediately in contact with the surface of the material causing a> 
decrease in concentration of solute in the bulk of solution, any con- 
_ clusions as to the causes of such decrease have been open to question. 
In the case of gelatin, however, it has been found possible to measure 
concentrations in the absorbed solution and this has thrown considerable 
light on the phenomenon known as adsorption. 

Adsorption is a term now widely used to indicate the removal of 
solute from solution by a material in contact with the solution. An 
empirical formula was proposed by Freundlich #® which agrees ap- 
proximately with some observed results over limited ranges, provided 
the two constants required in the formula can be selected to suit the 
findings. The formula may be represented as follows: 


Wossiax”, 


where w is the amount of solute removed from solution by unit quantity 
of the adsorbing material, x is the final concentration of solute, and 
a and b are constants selected to suit the occasion. Freundlich mentions 
that b may vary from o.1 to 0.5, but a very much more. 
' The very nature of the equation makes it capable of fitting a great 
variety of data, especially since the constants may be selected as de- 
sired, but it doesn’t explain anything. Referring back to Table XI, 
we find that the total quantity of chloride in the gelatin jelly at 
equilibrium, represented by V(y ++ z), can be represented as a function 
of the hydrogen-ion concentration by the use of Freundlich’s formula. 
Letting V(y + z) = 7.33x°-#?, we can plot a curve for the total quantity 
of hydrochloric acid, combined and uncombined, which has been ab- 
sorbed by the jelly that agrees fairly closely with both the calculated 
and observed results given in Table XI, although not quite so well 
as do the calculated and observed results with each other. Plotting 
logV(y +z) of the above equation against logx, we get a straight 
line, but the observed results never give a perfectly straight line, but 
vary in the same directions as do the calculated results of Table XI. 
The curve for the concentration of gelatin chloride shown in Fig. 42 
also can be represented approximately by Freundlich’s formula by 
letting z—o0.10x™*. The formula is a convenient means of represent- 


* Kapillarchemie. H. Freundlich, Leipsic, 1909. 


132 THE CHEMISTRY OF LEATHER MANUFACTURES 


ing a’reaction approximately over a limited range, which it is able to do 
merely because many variables give curves that are nearly parabolic 
in shape. | | 

Adsorption, so far as it pertains to gelatin jellies, is a manifesta- 
tion of chemical combination complicated by the separation of the 
solution into two phases. We see no reason for looking upon adsorp- 
tion by other materials in any different light. In the case of suspensoids, 
we are dealing with two phases of the solution apparently analogous 
to those of gelatin systems, the film of solution enveloping the particles 
corresponding to the solution absorbed by the jelly. 


For a more elaborate treatment of certain phases of modern theories 
of the physical chemistry of the proteins, the reader is referred to 
Loeb’s “Proteins and the Theory of Colloidal Behavior” ** and to 
Bogue’s “Chemistry and Technology of Gelatin and Glue.” *® 


-». 46 McGraw-Hill Book Co., New York, 1922. 


Chapter 6. 


Preservation and Disinfection of Skin. 


Practically every country in the world supplies hides and skins for 
leather manufacture. The skins from large, fully grown animals are 
usually called hides, those from half grown animals of the larger 
variety kips, while those from small or very young animals, or those 
intended for furs, are ¢alled skins. For example, as the calf grows 
into a cow, its skin remains a skin until it reaches a weight of about 
I5 pounds in the wet state, when it becomes a kip, while it becomes 
a hide at about 30 pounds. These figures are necessarily arbitrary, 
but serve to indicate the general scheme of classifying skins accord- 
ing to size. A bull hide may weigh more than 100 pounds. A sheep 
skin always remains a skin because it never assumes great size. The 
skin of the full grown East Indian buffalo is called a kip because it 
is smaller than the ordinary cow hide. For convenience, the term skin 
is used in its general sense throughout this book to include hides and 
kips, except when referring to specific cases. 

The fact that animals are generally raised and slaughtered for food 
rather than for purposes of leather manufacture makes the tanner’s 
chief raw material a by-product of the packing industry. For this 
reason a decreasing consumption of leather has but little influence upon 
the continued supply of skins, although it does tend to lower their 
market value. On the other hand, a brisk demand for leather generally 
does not in itself stimulate the raising and slaughtering of cattle, but 
rather has the effect of increasing the vigilance against damage to 
the existing supply of skins by putrefaction, careless handling, or the 
ravages of insects. Raw skins are highly putrescible and, since a 
considerable period of time usually elapses between the slaughter and 
the first tannery operation, it 1s necessary to subject them to some 
method of preservation as soon as possible after flaying. 


Salting. 


The commonest method of preserving skins, where they do not 
have to be transported very long distances and where salt is reasonably 
cheap and plentiful, is salting or curing, as it is sometimes called. 
The skins are laid out flat, flesh side up, and covered with salt in 
amount equal to about one quarter of their weight. Often they are 
placed in piles so arranged that the sides are higher than the center, 
which keeps the brine from flowing away, but this is undesirable unless 


133 


134 THE CHEMISTRY OF LEATHER MANUFACTURE 


the skins have previously been washed free from blood. Sometimes 
they are soaked in a concentrated solution of salt first and then covered 
with dry salt. The object is to get the salt to diffuse completely through 
the substance of the skins, which may require only a few days for 
light skins or weeks for heavy hides. Each skin is then folded up, 
hair side out, and in this condition sent to the market. 

Where the blood and lymph have been removed from the skins 
immediately after flaying and enough pure salt has been used to 
give a nearly saturated solution in the skins, putrefaction is reduced 
to an almost negligible degree and the skins may be kept for a long 
time with comparative safety. Common salt is most widely used, but 
sodium sulfate and other neutral salts are also effective and actually 
used in some places. 


Salt Stains. 


A defect commonly found in salted skins is the appearance of 
peculiar stains, usually either rusty brown or greenish blue in color, 
which are sometimes very difficult to remove and only become in- 
tensified and darkened through contact with sulfide-lime liquors or 
vegetable tan liquors, substantially lowering the market value of the 
leather. Because they are a source of loss and annoyance to the tanner, 
efforts have been made, from time to time, to determine their cause 


and methods for preventing them. Some stains disappear when the un- 


haired skins are pickled with a solution of sulfuric acid and salt, but 
others are resistant even to this process as ordinarily conducted. These 
stains received the name salt stains from the general belief that they 
were caused by the salt used in curing. At any rate, it was ap- 
preciated that their frequency of occurrence was influenced by the 
composition of the salt and the method of its application. 

The percentage of stained skins was especially high in those parts 
of Europe where edible salt is taxed and the salt used for curing must 
be denatured. The use of commercial aluminum salts, particularly 
those containing iron, was looked upon with suspicion and the scientific 
men of the industry began to seek other denaturing materials that 
would tend to prevent rather than to cause stains. 

One important school of thought regarded bacterial action as being 
largely responsible for the formation of the stains and sought de- 
naturing materials capable of checking bacterial growth. Paessler * 
found that the percentage of stains appearing on skins could be greatly 
reduced by curing with salt denatured with 3 per cent of its weight 
of anhydrous sodium carbonate. His discovery was put into general 
use and had the important effect of considerably decreasing the 
percentage of stained skins. : | 

Schmidt ? showed that bacterial action could be effectively checked 
by using salt previously sprinkled with a I2-per cent solution of zinc 
chloride and this method has been used to some extent to prevent salt 


1 Salting of Hides and Skins. J. Paessler. Ledertech, Rundschau (1912), 137. 
, ? Depreciation of Skins in Process. C. E. Schmidt. Shoe & Leather Rep., March 
» I9II. ; 


PRESERVATION AND DISINFECTION OF SKIN — 135 


stains. But, after making a series of comparative tests, Paessler * 
claimed that zinc chloride was no more effective than sodium carbonate 
in preventing salt stains. 

Romana and Baldracco * suspected the blood and lymph as the source 
of the stains and tried washing the skins very thoroughly after flaying 
and before adding the salt. On skins thoroughly washed they found 
no stains at all. They also found that the stains could be prevented 
by adding to the salt used in curing 1 per cent of its weight of sodium 
fluoride. 

- Kitner ° suggested that many stains are caused by delaying the salt- 
ing operation until bacterial action has already considerably advanced. 
He advised a more thorough elimination of water by heavily salting 
the skins, draining off as much brine as possible, and then resalting. 
The brine drained off carries with it proteins which are very susceptible 
to putrefaction. 

Yocum ® observed that salt stains occurred much more frequently 
in summer than in winter and were most abundant where the skins 
had had greatest contact with the air or had been kept for the longest 
period in the salted condition. Tests for iron were obtained on pieces 
of filter paper previously moistened with acetic acid and placed on 
the stains. Where stains still appeared on the finished leather, he 
obtained a test for iron in the stained, but not in the unstained parts. 
But iron was often found in the ash of fresh skins which showed 
no stains when tanned at once without salting. This seemed to in- 
dicate that the staining was due to a change in the condition of the 
iron present which enabled it to combine with the skin. He was able 
to produce stains on skins by treating them with hemoglobin and sug- 
gested that the hemoglobin of the blood might have been the source 
of the staining material. 

Becker 7 made extended studies of yellow, orange, and red stains 
on skins and isolated from them bacteria which, in pure cultures, were 
able to produce the corresponding stains. He also found that adding 
salt, up to 10 per cent of the weight of the skin, favored the action 
of these bacteria, while greater amounts retarded it. He warned against 
the use of an insufficient quantity of salt in curing, storing the skins 
in a warm, damp atmosphere, and of allowing dirt and filth to re- 
main on the skins. As a means of preventing these stains, he recom- 
mended dipping the skins in a 0.25-per cent solution of mustard oil, 
followed by the application of plenty of clean salt denatured with 
sodium carbonate. Not being able to reproduce the blue stains by 
bacterial action alone, he admitted that these might be due to chemical 
changes other than those involving bacteria. , 

The great stress placed upon the role played by bacteria in the 
formation of salt stains adds interest to the work of Abt,§ ® who main- 

8 Soda as a Denaturant for Hide Salt. J. Paessler. Ledertech. Rundschau (1921), 169. 

4 Salting of Hides and Avoidance of So-Called Salt Stains. C. Romana and G, Bald- 
racco. Collegium (1912), 533. 

5 Theory of Salt Stains. W. Eitner. Gerber (1913), serially. 

®Salt Stains. J. H. Yocum. J. Am. Leather Chem, Assoc. 8 (1913), 22. 

7 Salt Stains. H. Becker. Collegium (1912), 408. 

8 Origin of Salt Stains. G. Abt. Collegium (1912), 388 


_.® Microscopical Examination of Skin and Leather Applied to the Study of Salt Stains. 
Ibid. (1914), 130. 


136 THE CHEMISTRY OF LEATHER MANUFACTURE 


tained that most of the salt stains he had examined in France were 
not caused by bacterial action. Particularly bad cases of staining were 
traced to the presence of crystals of calcium sulfate in the salt used 
for curing. ‘The stains themselves always contained considerable quan- 
tities of calcium phosphate as well as iron. ‘The stained regions always: 
gave more intense qualitative tests for iron than the unstained regions, 
but analysis showed the same actual quantity of iron in both. He 
pictured the stain formation as follows: Calcium sulfate present in 
the salt used for curing is precipitated as phosphate through contact 
with ammonium phosphate derived from the nucleic acids of the skin. 
The ammonium sulfate thus liberated then reacts with insoluble ferrous 
carbonate, naturally occurring in the skin, forming the soluble ferrous 
sulfate, which forms a stain by combining with the skin protein. 

Abt attempted to follow the progress of the staining under the 
microscope and found that the cell nuclei disappear as the staining 
increases. ‘The connective tissues gradually disintegrate, but he could 
find no bacteria between the altered fibers, nor did the disintegration 
resemble the type of decomposition producd by bacteria. He thought 
the iron probably originated either in the chromatin of the cell nuclei 
or from the blood. A second type of stain contained no calcium phos- 
phate, but the epithelial cells were strongly pigmented. These stains 
he regarded as due to the fixation of the pigment by mineral matter 
in such a way as to prevent its decomposition by the lime liquors 
later on. Abt also recommended adding sodium carbonate to salt to 
be used for curing because it precipitates the calcium salts present and 
also exerts an antiseptic and dehydrating action. 

Although Abt contended that most of the stains which he had ex- 
amined were not caused by bacterial action, he admitted that bacteria 
might play an important part in the formation of other types of stains. 
In fact, he *° isolated an organism from one stain capable of producing 
a brown color on gelatin in the presence of traces of calcium phosphate 
and iron. 

At least three different explanations have been offered to account 
for the effectiveness of sodium carbonate in preventing salt stains. 
Abt attributed it to the precipitation of calcium salts which might be 
present in the salt used for curing. Paessler and others looked upon 
it as due to the production of an alkalinity unfavorable to the action 
of the bacteria thought to be responsible for the stains. Moeller,?+ - 
however, suggested that the staining is a tanning action, due to such 
agents as the melanins or to iron and sulfur bacteria, but that this 
tanning action cannot proceed in alkaline solution. It is, of course, 
obvious that the sodium carbonate has the important effect of preventing 
iron salts from passing into solution, in which condition they would be 
free to combine with the skin forming the stains. 

Summing up the work of various investigators, it would appear 
that salt stains are of several kinds and inay be produced directly by 
bacteria, such as Becker’s chromogenic organisms, or by soluble iron 


70 Role Played by Bacteria in Production of Salt Stains. Collegium (1091 204. 
1 Origin of Salt Stains. W. Moeller. Collegium (1917), seriall Core, 


fresh kVATION AND DISINFECTION OF SKIN 137 


salts. These iron salts may be introduced in the salt used for curing 
or may be formed from the insoluble iron salts already present in the 
skin, either by chemical action, as described by Abt, or through the 
intervention of bacteria.- The blood and lymph of skins furnish, an 
excellent medium for bacterial growth and contain compounds of both 
iron and phosphates. 

The following simple rules represent the best means known to the 
author for preventing these undesirable stains and, it would seem, 
ought to be quite effective, if carefully observed at the point of slaugh- 
ter. Immediately after flaying, the skins should be washed very 
thoroughly in running water to remove as much blood, lymph, and 
other soluble matter as possible and then salted uniformly in all parts 
with plenty of clean salt, free from iron and containing about 4 per 
cent of its weight of anhydrous sodium carbonate. During the time 
required for the salt to diffuse completely through the skins, they 
should be kept in a cool place and the brine formed should be allowed 
to drain away, carrying with it any soluble proteins not previously 
washed out. Salt equal in amount to at least one quarter of the weight 
of the skins should be used. Proper curing of skins is necessary, 
not only: to prevent the formation of stains, but also to prevent 
putrefaction that would otherwise impair the yield and substance of the 
leather. 


Drying. 


In tropical countries, like Java and India, from which skins are 
often transported very long distances, the simplest and most economical 
method of preserving skins is to dry them. ‘This is true for all regions 
where salt and antiseptics are scarce. Moreover, drying reduces the 
weight of the skin by about 70 per cent. In the absence of moisture, 
putrefactive bacteria are practically without action on the skin proteins, 
although the drying does not always kill the bacteria. 

When this method of preserving skins is intelligently controlled, 
very little damage to the skin results. In hot climates, care must be 
exercised to prevent excessive heating of parts of the skin which are 
still wet or the protein matter may decompose. Sometimes skins are 
dried so rapidly that the outer layers feel quite dry, while the interior 
is still moist enough to permit putrefaction. Skins packed and shipped 
in this condition are liable to considerable damage. Defects of this 
kind usually cannot be detected until the tanner attempts to soak the 
skins back, when they may actually disintegrate or the grain and flesh 
layers may tend to separate, due to the hydrolysis of the protein matter 
in the interior. If the drying has been unduly prolonged at high 
temperatures, the tanner may have considerable difficulty in soaking 
the skins back to their normal water content. 

The skin tissues continue to live for some time after the death of 
the animal and, in the living condition, are not readily subject to 
putrefaction. It is therefore desirable to dry skins as soon as possible 
after flaying. ‘They should first be cleansed thoroughly by washing 
away all the blood and lymph and then suspended freely in a current 


138 THE CHEMISTRY OF LEATHER MANUFACTURE 

of cool air until dry. Where conditions are such that drying cannot 
be effected sufficiently rapidly to prevent putrefaction, as in damp 
climates, it is customary to treat the skins first with some antiseptic, 
such as naphthalene, which acts also to protect the skins against the 
attacks of insects during drying. 

The advantages of drying, as a means of preserving skins, are 
simplicity and speed of operation, independence of a supply of pre- 
servative material, and low transportation costs for the skins. The 
disadvantages are the difficulty of wetting the skins back later to their 
normal water content, the almost impossibility of detecting damage 
to the skin proteins until they are wet back, and the fact that dried 
skins may carry disease-producing bacteria or their spores in a form 
likely to spread infection. 


Salting and Drying. 


Sometimes the methods of salting and drying are combined to ad- 
vantage. ‘The skins are first salted in the usual manner, the brine is 
allowed to drain away, and they are then allowed to dry slowly. The 
salt has the effect of hindering putrefaction during the drying. 

This method is extensively used in some parts of India, but the 
salt used is a native earth which, according to Procter,!? consists chiefly 
of sodium sulfate mixed with sand containing insoluble compounds 
of iron and aluminum. This material is made into a very thin paste, 
which is brushed onto the flesh side of the skins. Next day more of 
the paste is rubbed onto the flesh side of the outstretched skin and 
rubbed into it with a porous brick. After 3 or 4 saltings, the skins 
are dried under cover and are ready for export. The iron present in 
the salt sometimes causes a staining of the skins when they are kept 
for a long time in a moist atmosphere. 


Pickling. 


Skins may be preserved by pickling in a solution of sulfuric or 
hydrochloric acid and sodium chloride. A solution made about N/20 
as to acid and 2N as to salt is efficient. This method is not in general 
use for fresh skins because of the complications involved in attempt- 
ing to bring them into an alkaline condition later on for unhairing. 
But for sheep skins, already dewooled, it is a widely used method and 
convenient, because the skins are then ready for chrome tanning with- 
out further treatment. 

The value of this method for preserving sheep skins is increased 
by the fact that wool is often more valuable than the skin. The skins 
are frequently purchased by wool pullers, who remove the wool by 
methods to be described in Chapter 8, and then lime, bate, and pickle 
them, in which condition they are stored or resold to tanners. ‘This 
method of preservation permits the immediate use of the wool without 
destroying the skin or forcing it directly into the tanning process. 


#2 Principles of Leather Manufacture, 2nd edition. H. R. Procter. D. Van Nostrand 
Co., New York. * 


PRESERVATION AND DISINFECTION OF SKIN _ 139 


In pickling, the skins are usually thrown into a vat, equipped with 
a paddle wheel for keeping the liquor and skins well stirred and con- 
taining a strong solution of salt with a definite excess of sulfuric acid, 
which is controlled by analysis. The skins are left in the pickle liquor 
until equilibrium has been practically reached, which is determined by 
noting when there is little further decrease in concentration of acid 
with time. This may require anywhere from 4 to 24 hours, depending 
upon the thickness and condition of the skins and upon the equilibrium 
concentration of acid selected. Equilibrium is reached more quickly 
when more concentrated solutions of acid are used, but, if too strong 
a solution is used, it may be necessary to remove some of the acid 
prior to tanning by washing the skins in a concentrated neutral salt 
solution. After pickling, the skins are allowed to drain and are then 
stored in a damp condition until the tanner is ready to put them into 
process. 


Disinfection. 


Infectious diseases among cattle are common in many countries, 
particularly in Asia. For this reason some kind of disinfection of 
skins to be transported from infected areas is necessary in order to 
prevent the spread of disease germs. Much attention has been paid to 
preventing the spread of rinderpest,. foot-and-mouth disease, and. the 
much dreaded anthrax, which occasionally proves fatal to human beings 
infected with it. Various governments have issued rules to be fol- 
lowed in disinfecting skins from regions known to be infected. The 
greatest precautions have been directed against the spread of anthrax 
because of the danger to human life, but any treatment effective against 
this disease may be considered effective against the others as well. 

Anthrax is the disease caused by the spore-bearing bacillus anthracis. 
The bacillus possesses a short rod-like form and is easily destroyed. 
According to Seymour-Jones,’? drying alone will kill the rod bacillus. 
The spore, on the other hand, is very resistant to methods of disinfec- 
tion that do not cause some injury to the skins, and it is this that makes 
the problem of disinfecting skins a difficult one. Anthrax spores have 
been found in dried skins and in blood clots on hair and wool, but 
seldom, if ever, in wet salted skins. 

Practical methods of disinfection are limited because so many disin- 
fectants are injurious to the skin and reduce its value for leather 
making. Consequently only a few workable methods have been devised. 
Of these, the best known is that of Seymour-Jones,'* who recommends 
its employment at the point of export rather than of import because 
of the danger of spreading the disease during transit. It consists in 
soaking the dried skins for from I to 3 days in a I-per cent solution 
of formic acid containing 0.02 per cent of mercuric chloride. They 
are then soaked for an hour in a saturated solution of common salt, 
drained, and baled for shipment. 

18 Anthrax Prophylaxis in the Leather Industry. Alfred Seymour-Jones. J. Am. Leather 


Chem, Assoc. 17 (1922), 55. a 
144 Formic-Mercury Anthrax Sterilization Method. Alfred Seymour-Jones, London (1910). 


140 THE CHEMISTRY OF LEATHER MANUFACTURE 


Procter and Seymour-Jones?* studied the rate of absorption of 
formic acid and mercuric chloride during the soaking operation at a . 
number of different concentrations, using 1 liter of solution per 100 
grams of dried skin. The concentration of acid in the solution always 
fell slowly during a period of 20 hours, but that of the salt at first 
increased and then dropped, finally approaching a limiting concentra- 
tion. The initial increase in concentration of mercuric chloride was. 
found to be the result of a greater initial rate of absorption or pene- 


Moles Mercuric 

Chloride per 
100,000 
Liters, 


Concentration of Li quor 


2 4 6 8 LOG ete 14 16 ee 20 
Hours of Contact of Skin and Liquor 


Fic. 60—Change in composition of solution with time in the Formic-Mercury © 
Process for sterilizing skins. 


tration of water and acid than of the salt. The results of one of their 
experiments are shown in Fig. 60. a 
The absorption of water caused by the acid renders the skin almost 
as soft as in the fresh state and the subsequent immersion in saturated 
sodium chloride solution brings it into a condition resembling that of 
salted skins. Seymour-Jones points out that skins in this condition 
are not only properly disinfected, but that they present less of a gamble 
to the tanner because they show any defects in the skin that would not 
be visible when the skin is in the dried state. aS 
Schattenfroh ‘® proposed a method of disinfection involving the 
© Seymour-Jones Anthrax Sterilization Method. H. R. Procter and Arnold Seymour- 


Jones. Leather Trades Review through J. Am. Leather Chem. Assoc. 6 (1911), 85. 
1® A Harmless Method for the Disinfection of Skins against Anthrax. A. Schattenfroh. 


Collegium (1911), 248. 


eee pee tON AND DISINFECTION OF SKIN 14! 


soaking of infected skins in a solution containing 10 per cent of sodium 
chloride and 2 per cent of hydrochloric acid at 40° C. for 3 days. Much 
debate has waged over the relative merits of the Seymour-Jones and 
Schattenfroh methods. Tilley,’ after experimenting with both 
methods, concluded that the Seymour-Jones process is effective, but 
only provided the concentration of mercuric chloride is as high as 0.04 
per cent and the skins are not subjected within a week to treatment 
with sodium sulfide or other substance that would neutralize the disin- 
fectant. It should, therefore, be effective where the disinfection is 
carried out at a foreign port before shipping. Seymour-Jones,'* in 
reply, pointed out that neutralization of the disinfectant by sodium 
sulfide would take place only in the unhairing process, whereas, under 
conditions existing during this process, the sodium sulfide itself is a | 
perfect sterilizer of anthrax spores. This would seem to eliminate any 
possible danger of anthrax infection from skin or leather that had 
passed through the usual lime and sulfide method of unhairing. 

Tilley found the Schattenfroh method effective when the hides were 
allowed to remain in the acid-salt solution for 48 hours or longer. 
Schnurer and Sevcik,?® however, applied the Schattenfroh process to 
very heavy hides and obtained 4 positive tests of infection out of 11 
made after the hides had been in a solution containing 2 per cent of 
hydrochloric acid and Io per cent of sodium chloride for 72 hours. 
They attributed the more favorable results obtained by Schattenfroh to 
the fact that he experimented with very thin skins. Using the Sey- 
mour-Jones process on very heavy hides, they found it. necessary, in 
order to get complete sterilization in 24 hours, to increase the concen- 
tration of mercuric chloride to 0.2 per cent, but hides so treated were 
found by Eitner not to have suffered for tanning purposes. They also 
found it necessary to degrease heavy sheep skins before applying the 
Seymour-Jones process, as otherwise a ten-fold dose of mercuric 
chloride was required. 

Seymour-Jones objected to the Schattenfroh method on the ground 
that it is workable only under laboratory conditions and that its factors 
of time, temperature, and general manipulation are not suited to prac- 
tical operations. Ponder,?? investigating methods of disinfection for 
the Leathersellers Company of London, and Abt,*4 of the Pasteur 
Institute, Paris, working for a syndicate of French tanners, both re- 
ported in favor of the Seymour-Jones process. Apparently neither 
process does any injury to the skins that can be detected in the finished 
leather, according to the findings of numerous investigators. 

Abt, however, has pointed out that hides would contain no anthrax 
spores, if they were dried in the sun immediately after flaying, and this 
view is supported by Seymour-Jones. 

17 Bacteriological Study of Methods for the Disinfection of Hides Infected with Anthrax 
Spores. F. W. Tilley. J. Am. Leather Chem, Assoc. 11 (1916), 131. 

18 The Formic-Mercury Process for Sterilizing and Curing Dried Hides. Alfred Sey- 
mour-Jones. J. Am. Leather Chem, Assoc. 12 (1917), 68. 

1® Anthrax Disinfection of Hides. J. Schnurer and F. Sevcik. Tierdrztliches Zentralbl. 
through J. Am. Leather Chem. Assoc. 8 (1913), 174. 


20 A report to Worshipful Company of Leathersellers, 1911. C. Ponder. 
21 Disinfection of Anthrax Infected Hides and Skins, Pasteur Institute, 1913. G. Abt. 


Chapter 7. 
Soaking and Fleshing. 


As received at the tannery, skins contain much material unsuitable 
for leather manufacture and which would introduce serious complica- 
tions, if not removed as early in the process as possible. For this rea- 
son every effort is made to remove each undesirable constituent as soon 
as it can be done efficiently. The preparation of skin for tanning is 
carried out in a department of the tannery known as the beamhouse and 
includes, not only the removal of the undesirable parts, but also the 
regulation of the degree of swelling of the skin proteins. 

Ears, cheeks, hoofs, and tails are trimmed from skins still pos- 
sessing them and the flesh, or adipose tissue, is removed by working 
the skin in a fleshing machine, which forces the flesh side of the skin 
against a revolving roller set with sharp blades, which cut away the 
adipose layer. The trimmings and fleshings make up the tannery by-. 
product known as glue stock and are disposed of for manufacture into 
glue and gelatin. 

On the hair side of the skin, the epidermis is made up of a network 
of membranes, forming the walls of the epithelial cells, impermeable 
to the soluble proteins of the skin as well as to other material having 
large molecules or consisting of aggregates of molecules, while on the 
flesh side the adipose tissue consists of layers of fat cells bound to- 
gether by extensive series of semi-permeable membranes. It will, 
therefore, be readily appreciated why the adipose tissue must be removed 
before the skin can be thoroughly cleansed and freed from soluble 
protein matter. ; 

The collagen fibers of the skin are joined together at the lower 
boundary of the derma in such manner as to give increased strength 
to the skin. In fleshing, it is important to remove all of the adipose 
tissue without cutting into the derma, which would weaken its structure 
as well as lower the leather yield. But reference to Fig. 7 will show: 
that this is not difficult where the skin is in its normal state. The 
lower boundary of the derma is sharply defined and the adipose tissue 
is not joined securely to it at all points. But where the skin has under- 
gone partial or complete drying, satisfactory fleshing becomes a more 
difficult operation. 

During the ordinary methods of drying, protein jellies suffer a 
change of shape, as well as of size, depending upon their initial shape, 
the resistance offered to shrinkage in any direction, the rate of drying, 
and many other factors. This was prettily illustrated by Sheppard 

142 


SOAKING AND FLESHING 143 


and Elliott! with blocks of gelatin jellies. The photographs shown in 
Figs. 61 to 64 were kindly furnished by Dr. S. E. Sheppard of the 
Eastman Kodak Co. Fig. 62 shows four stages in the drying of a 
cube of 20-per cent gelatin jelly which was freely suspended in the 
air. No. 1 represents the original block of jelly, Nos. 2 and 3 inter- 
mediate stages in the drying, and No. 4 the dried block. At first the 
drying naturally proceeds most rapidly at the corners, or trihedral 
angles, and the faces of the cube become curved outward, as shown in 
No. 2, giving convex surfaces under tension. This is rapidly followed 
by the drying and hardening of the edges, forming a rigid framework, 
so that the bulk of the jelly now behaves as though suspended inside 
of a rigid wire frame. The faces now gradually recede and the edges 
become somewhat incurved until a sort of inner cube is formed with 
connected flanges reinforcing it, any cross-section through this having 
an J-beam structure, as though the drying proceeded in a manner 
developing the greatest resistance to stress. The flange-like edges 
appear to form sections of hyperboloids with a common focus at the 
center of the cube. Fig. 61 shows three stages in the drying of a 
sphere of gelatin jelly. Even here the drying is not uniform, but the 
surface becomes puckered and wrinkled. 

The dried forms of two cylinders of gelatin jelly are shown in 
Fig. 64 and their end views in Fig. 63. One base of the first and 
both bases of the second cylinder were allowed to adhere to rigid sur- 
faces during the drying. The shrinkage in area of these bases being 
prevented, the reduction in volume had to be compensated by greater 
shrinkage in other directions. In the drying of a thin coat of gelatin 
jelly on a glass plate, the shrinkage takes place almost entirely in the 
direction perpendicular to the plane of the glass surface. 

Upon soaking dried blocks of gelatin in water, the swelling pro- 
ceeds in the direction counter to that followed during drying and 
the blocks tend to assume the shapes and sizes they possessed before 
drying. 

During the drying of skin, the distortions of shape suffered by the 
insoluble protein constituents are further complicated by the tendency 
for the fibers to adhere to each other. Before a skin can be fleshed 
satisfactorily, it is necessary to soak it in water long enough so that 
all of the insoluble protein constituents may swell to their normal sizes 
and shapes. When the skin is not uniformly swollen, the boundary 
between the derma and adipose tissue cannot be made to lie in a single 
plane. The fleshing machine would then cut the skin so as to leave 
the flesh side apparently smooth, but in so doing would either leave a 
considerable amount of adipose tissue on the skin to interfere with 
the proper cleansing of the skin or else injure the skin by cutting into 
the derma. The flesh side would look smooth enough upon coming 
from the machine, but would be ragged and irregular in thickness after 
the skin had been soaked further or swollen in the liquors used later. 
F. L. Seymour-Jones says that in Europe it is customary not to flesh 


1The Drying and Swelling of Gelatin. S. E. Sheppard and F. A. Elliott. J. Am. Chem, 
Soc. 44 (1922), 373. 


Fig. 61.—Three Stages in the Drying of a Sphere of Gelatin Jelly. = 
=e i ee ee 


ae 


Fig. 64.—Two Cylinders of Gelatin Jelly Dried with One and Two 
Faces, Respectively, Adhering to Rigid Surfaces. 


144 


SOAKING AND FLESHING 148 


skins until after at least a preliminary liming. In America, tanners of 
goat skins usually flesh them after liming. 

Heavy, dried hides not only require a more drastic treatment than 
light, fresh skins, but are also better able to stand it without injury 
to the resulting leather. In order to get better and more uniform 
results, the tanner sorts the skins he receives according to weight and 
general condition. A suitable number of skins, all as nearly alike as 
possible, are assembled into a unit lot and kept together throughout 
the process. The treatment is then determined by the average size and 
condition of the skins as well as by the kind of leather desired. Very 
large hides are often cut into two sides along the line of the back bone, 
for convenience in handling. 

Where the skins come to the tannery in a perfectly fresh condition, 
the soaking and fleshing operations are extremely simple. After the 
skins have been trimmed, the adhering blood and dirt are removed by 
tumbling the skins for half an hour or more in an open drum through 
which water is flowing. They are then fleshed, after which they are 
soaked in several changes of clean, cold water containing salt or a 
small quantity of alkali, the object of which is to free them from 
soluble protein matter that would otherwise coritaminate the liquors 
used to loosen the hair and epidermis. The purpose of the salt, or 
alkali, is to render the globulins soluble so that they may be removed 
along with the albumins. 

For dried, or partially dried, skins it 1s necessary to soak the skins 
both before and after the fleshing operation. The first soaking is pri- 
marily for the purpose of swelling the insoluble proteins back to their 
normal sizes and shapes so that the fleshing operation may be carried 
out efficiently. The second soaking is for the purpose of freeing the 
skin from soluble protein matter. 

The time required for the first soaking depends upon the extent 
to which the skins have been dried. Completely dried skins absorb 
cold water extremely slowly. Since skins, as received at the tannery, 
are almost invariably contaminated with proteolytic bacteria, the use 
of warm water in soaking is somewhat risky, unless the process is very 
carefully watched. It is usually preferable to hasten the swelling of 
dried skins by adding small quantities of acid or alkali to the soak 
waters. 

Because of the attention centered on the Seymour-Jones process of 
disinfecting skins, described in the preceding chapter, formic acid has 
often been used as a swelling agent, although other acids can be used 
equally as well by applying a simple system of chemical control. Alka- 
_ lies, however, are more suitable where the skins are subsequently to be 
treated with alkaline liquors to loosen the hair. Sodium sulfide is most 
commonly employed to swell dried skins because it requires less careful 
control than the use of more caustic materials, such as sodium hydroxide. 
In soaking, a gallon of water is usually used per pound of wet skin or 
for one-fifth of a pound of completely dried skin. Making the initial 
concentration of alkali about 0.02 normal is usually enough to initiate 
the swelling without causing damage either to the skin cr the hair. The 


146 THE CHEMISTRY OF LEATHER MANUFACT Oe 


solution after using is then only very faintly alkaline, the greater por- 
tion of the alkali having combined with the protein matter. The 
alkaline liquor is used only for the first soaking after which the skins 
are moved into fresh water each day until swollen to normal. 

Sometimes the absorption of water and softening of the skins is 
assisted by tumbling them in revolving drums with water between suc- 
cessive soakings. This is usually done with heavy, dried hides or 
sides. 

As a rule, salted skins can be fleshed after soaking for only one 
day, or less. After fleshing, it has been the custom to soak the skins 
in successive changes of water until practically all of the salt has been 
removed. ‘The salt diffuses out from the skin much more rapidly than 
the soluble protein matter, so that continuing the soaking until all of 
the salt has been removed is not unduly prolonging the process where 
it is desirable to free the skin as far as possible from soluble protein 
matter. This custom, however, has created a widespread, but erroneous, 
impression that it is dangerous to carry salt into the lime liquors. On 
the contrary, salt assists in the unhairing and plumping of skins by the 
ordinary lime liquor. Its action in this respect appears to be due to 
the fact that it increases the hydroxide-ion concentration of alkaline 
solutions in general.’ 7 : 

Defects in finished leather are often traceable to the soaking opera- 
tion. Although bacterial action is the chief source of danger, the skin 
may suffer from other causes. The tissues of the body do not neces- 
sarily die with the animal, but may continue to live for an indefinite 
period, if sufficiently well supplied with nourishment. For this reason 
it is conceivable that the sudden chilling of a fresh skin may exert an 
effect upon the muscles and glands of the thermostat layer. If, for 
example, the erector pili muscles were suddenly contracted and para- 
lyzed by chilling, the result would be a permanent roughening of the 
surface of the skin. There have been cases where an unusual roughness 
of the grain surface of leather seemed to result from the sudden im- 
mersion of the warm skins, before tanning, in water near the freezing 
point. But the danger from proteolytic bacteria makes the use of warm 
water undesirable for soaking. Cold water should be used, but the 
operations should be so conducted that the temperature of the skins falls 
eradually. : 

How long the various parts of the skin continue to live and function 
after the animal has been flayed remains to be determined. We do 
know, however, that the skin undergoes changes of one sort or another 
practically from the moment of flaying. McLaughlin? noted that the 
rate of swelling of hide in saturated lime water decreases during the 
first two or three hours following the flaying of a freshly killed animal. 
A strip of hide put into lime water containing undissolved lime in 
excess 30 minutes after flaying swelled about 30 per cent more in 120 


? The Hydrogen- and: Hydroxyl-Ion Activities of Solutions of Hydrochloric Acid, Sodium 
and Potassium Hydroxides in the Presence of Neutral Salts. H. S. Harned. J. Am. Chem. 
Soc. 37 (1915), 2460. 

3 Post-Mortem Changes in Hide. G. D. McLaughlin. J. Am. Leather Chem. Assoc. 
16 (1921), 435. ; ; hee 


SOAKING AND FLESHING 147 


hours than a corresponding strip put into the lime water 210 minutes 
after the flaying. 

This is, of course, not surprising in view of the fact that many 
changes are known to occur in skin, after the death of the animal, all 
of which would tend to retard the swelling in lime water. The coagu- 
lation of the blood, during which fibrinogen is converted into fibrin, 
would tend to retard the penetration of lime into the skin and the 
partial drying of some of the tissues would act in a similar manner. 
Decomposition of some of the protein constituents would yield simpler 
bodies capable of forming salts of calcium, which would serve to 
repress the swelling of the proteins by calcium hydroxide. It is pos- 
sible also that some of the proteins capable of swelling are gradually 
broken down into simpler bodies not having the power to swell. 

Where the preservation of a skin has been done carefully and 
intelligently, these changes appear not to have any detrimental effect 
upon the leather produced. The author has tested this by comparing 
the tannage of skins properly preserved and kept for months before 
tanning with the tannage of skins put into process within an hour of 
the death of the animals; no appreciable differences could be detected 
by chemical, physical, or microscopical examinations of the final leathers. 
But where there is carelessness in handling, the skins may suffer irrep- 
arable damage before the soaking operation has been completed. 

The commonest source of danger in soaking is bacterial action. 
Although the inner surface of the skin on the living animal may be 
free from bacteria, it acquires them from the atmosphere very rapidly 
from the instant of flaying and acts as an ideal medium for the repro- 
duction of bacteria. By the time the skin reaches the soak vats, it is 
usually contaminated with countless millions of bacteria. Many species 
of these bacteria are known to secrete enzymes, which may prove as 
harmful as the bacteria themselves. The chief practical object to be 
gained from a study of the bacteria common to tannery soak waters is 
to find means of destroying them, or at least of preventing them from 
doing any damage to the skins. An extensive series of investigations 
of the bacteria and enzymes present in tannery liquors has been made 
by Wood.* 

Andreasch * isolated a number of species of bacteria from tannery 
soak liquors of which he identified the following: 


Bacillus fluorescens liquefaciens (Fliigge). 
. megaterium (de Bary). 

. subtilis. 

. Mesentericus vulgatus. 

mesentericus fuscus. 

. mycoides (Fltgge). 

. liquidus (Frankland). 

. gasoformans (Eisenberg). 


SoleclusMecleclesles 


* Properties and Action of Enzymes in Relation to Leather Manufacture, J. T. Wood, 
J. Ind. Eng. Chem. 13 (1921), 1135. ; 
° Der Gerber, 1895-6; J. Soc. Chem, Ind., 1896-7. 


148 THE CHEMISTRY. OF LEATHER MANUPAGIR 


White bacillus (Maschek). 

Proteus vulgaris. 

Proteus mirabilis. : 
B. butyricus (Hueppe). 

White streptococcus (Maschek). 

Worm shaped streptococcus (Maschek). 

Grey coccus (Maschek). 


Fig. 65.—Typical Plate Culture on Gelatin of Soak Water Used for 
Softening Dried Sheep Skins. 


All these may be classed as putrefactive organisms that secrete a 
variety of enzymes, many of which act energetically on hide substance. 

Fig. 65, taken from Wood’s paper, shows a typical plate culture 
on gelatin of a soak water used for softening dried sheep skins, in 
which no chemicals were used. The development of the colonies had 
to be stopped by the application of formaline vapor before many of 
the species had time to develop; otherwise the whole plate would have 
been liquefied. 


SOAKING AND FLESHING 149 


Rideal and Orchard ® examined the action of B. fluorescens lique- 
faciens on gelatin to which had been added Io per cent of Pasteur’s 
solution to serve as nutrient medium. The gelatin was completely 
liquefied in three and one-half days. It was shown that the liquefac- 
tion of the gelatin was due to an enzyme secreted by the bacteria. The 
liquefied gelatin was alkaline and had a slight odor suggesting putre- 
faction, but contained no hydrogen sulfide. A notable feature was 
the small amount of ammonia and volatile bases produced; only 0.2 
gram of ammonia per 100 cubic centimeters was produced even after 
16 days’ incubation. 

In bacterial action of a certain type, one of the first effects to be 
noticed is the loosening of the hair, a condition known to the trade as 
hair-slippiness. Either the bacteria, or the enzymes which they secrete, 
act upon the soft epithelial cells of the Malpighian layer of the epider- 
mis, liquefying them and thus effecting a separation of the whole of 
the epidermis and hair from the rest of the skin. This action alone 
is not harmful, but the bacteria develop rapidly and soon begin to 
attack the fibers in the grain surface and the skin is permanently 
injured. ‘This effect shows itself in the finished leather in the form 
of dull spots, or what is known as pitted grain. In some cases the 
bacteria attack the heavier collagen fibers without injuring the fibers 
of the grain surface. When the bacteria attack the proteins of the 
thermostat layer, they weaken the connection between the fibers of the 
grain surface and those of the reticular layer; in the finished leather 
the grain surface then tends to peel off and its looseness of connection 
with the main body of the skin gives it the appearance known as 
pipy grain. : 

Chemists not familiar with the chemical composition of fresh skin 
sometimes fall into the error of assuming that the presence of nitroge- 
nous matter in a used soak liquor indicates that the collagen fibers have 
been attacked. One of the objects of soaking skins is to remove the 
soluble proteins so that they will not be carried forward to contaminate 
the liquors used to loosen the hair. 

Bacteria may become lodged just under the grain surface of the 
skin and resist the action of the various liquors through which the 
skin passes. They then become the source of many most annoying 
troubles. They may produce dull spots or stains or hydrolyze the fats 
used later to soften the leather. Hydrolyzed and oxidized fats are the 
_ common sources of spews appearing on the surface of finished leather. 

In the use of what is known as the putrid soak, bacteria are put to 
work by being made to assist in the softening of dried hides. But 
this method is not only an obnoxious one, but one so difficult to control 
that some damage very often accompanies the softening action. The 
method is seldom used in modern countries, but in some parts of India 
dried skins are softened by soaking them in putrid pools of liquor 
containing all kinds of tannery refuse. 

In most tanneries, no attempt is made to utilize the bacteria of the 
soak waters. On the contrary all practical means available are used 

® Analyst, Oct., 1897. 


10 THE CHEMISTRY OF LEATHER MANUFACTURE 


to prevent bacterial action in the soaking operation. In a study of the 
effect of hydrogen-ion concentration upon the activities of putrefactive 
bacteria, the author has found that they are most active between the 
pH values 5.5 and 6.0. This probably explains the value of using 
alkaline soak waters; the liquefaction of skin by bacteria at a pH value 
of 5.5 is usually greatly retarded or even completely checked by raising 
the pH value to 12. A similar effect is observed by lowering the pH 
value to about 3 by the addition of acid. 

Procter’ has pointed out the advantages of using sulfurous acid 
in the soak waters. It assists in the absorption of water by the skin 
and at the same time prevents bacterial action. He found that no 
putrefaction takes place, even if the skins are later retained for a con- 
siderable time in water, and the acid has little or no solvent effect on 
the collagen fibers, whose strength is well preserved. 

Alkalies are about equally effective as acids both in the softening 
of dried skins and in checking bacterial action and are generally pre- 
ferred because they assist rather than retard the action of the lime 
liquors in loosening the hair. 3 

Aside from the use of acids and alkalies, the chief precaution taken 
against bacterial action in the soaks is the use of plenty of clean, cold 
water. If the temperature of the water is not allowed to rise above 
1o° C. and plenty of clean water is used, the skins are not likely to 
' suffer any serious damage from the soaking operation itself. 


‘Principles of Leather Manufacture, Second Edition (1922), 16r. 


Ghapter so: 
Unhairing and Scudding. 


After the skins have been trimmed, cleansed, freed from adipose 
tissue and soluble matter, and have again become soft through absorp- 
tion of their normal water content, they are ready for the series of 
operations involved in the removal of the epidermal system. It will be 
recalled from Chapter 2 that this system includes the epidermis, hair, 
and the sebaceous and sudoriferous glands and differs from the true 
skin under it in origin, structure, method of growth, and chemical com- 
position. The several parts of the epidermal system differ markedly 
in their resistance to chemical reagents and it is rather fortunate for 
the tanner that the part most readily digested is the portion of the 
Malpighian layer resting on the grain surface. When the epithelial 
cells of this layer are destroyed, the rest of the epidermis and the hair 
become completely separated from the true skin and can easily be 
removed mechanically. 


Sweating. 


What is probably the oldest method known for unhairing skins 
received the name sweating from the nature of the process in its more 
highly developed state. It consists of little more than the putrefaction 
of the cells of the Malpighian layer. Since it is only necessary to allow 
a fresh skin to remain for a day or two in a warm, damp place to cause 
a loosening of the hair, the method was probably discovered very early 
in the history of the human race. It is not improbable that the acci- 
dental discovery of this action first revealed to the ancients the advan- 
tages of unhaired skins for certain purposes. 

‘Because of the danger of serious damage to the skins in the sweat 
chambers, unless the process was very carefully watched and controlled, 
it ceased to be popular for the best grades of skins after safer methods 
of unhairing were devised. It is still in use in some tanneries for the 
lower grades of skins, such as the cheaper classes of dried hides and 
sheep skins where the wool is valued more highly than the skin. 

The skins are generally hung from beams in a closed room in which 
the air is kept warm. and humid. The temperature, humidity, and 
ventilation must be carefully controlled. During the process a con- 
siderable quantity of ammonia is evolved and this assists in the unhair- 
ing action. Just as soon as the hair slips easily, the skins are removed 
from the sweat chamber and dumped into saturated lime water. The 
lime water serves to retard further bacterial action and to cause the skins 

151 


Fig. 66.—Vertical Section of Sheep Skin. 
(After 42 hours in sweat chamber.) 


Location: butt. Eyepiece: none. 

Thickness of section: 20 wu. Objective: 16-mm. 

Stains: Van Heurck’s logwood, Wratten filter: H-blue green. 
Daub’s bismarck brown. Magnification: 45 diameters. 


152 


Fig. 67.—Vertical Section of Thermostat Layer of Sheep Skin. 
(After 42 hours in sweat chamber.) 


Location: butt. Eyepiece: 5X. 

Thickness of section: 20 Uw. Objective: 16-mm. 

Stains: Van MHeurck’s logwood, Wratten filter: H-blue green. 
Daub’s bismarck brown. Magnification: 135 diameters, 


153 


* 


154 THE CHEMISTRY OF LEATHER MANUFACTURE 


to swell somewhat by absorption of water; the skins upon coming from 
the sweat chamber are in a very flaccid and slimy condition. 

Wilson and Daub? recently made a study of the sweating process 
under the microscope. Pieces of fresh sheep skin were kept in a closed 
receptacle having an atmosphere saturated with water vapor at 38° C. 
At frequent intervals strips of skin were removed for sectioning and 
examining under the microscope. At the end of 42 hours, the wool 
could be rubbed off with ease and the skin had apparently suffered no 
damage. The odor of ammonia in the receptacle after the first day was 
very pronounced. | 

The first sign of action visible under the microscope was the sepa- 
ration of the cells of the Malpighian layer from one another and from 
the surface of the derma. This action gradually spread to the outer- 
most layers of cells of the sebaceous and sudoriferous glands. On the 
second day the action had proceeded so far that the epidermis, glands’ 
and wool were completely separated from the derma and many of the 
epithelial cells had completely disintegrated. A section of the skin after 
being in the sweat chamber for 42 hours is shown in Fig. 66. The 
upper portion of the section is shown in Fig. 67 at a much higher 
magnification. 

It will be noted that the corneous layer is still intact, but the Mal- 
pighian layer has almost completely disintegrated, the linings of the 
hair follicles are broken up, and the glands have all been loosened and 
separated from the derma. Fig. 66 should be compared with Fig. 28, 
which represents a section from the same skin fixed in Erlicki’s fluid 
within an hour after the death of the animal. 

In practice, the systematic cleaning of the sweat chambers is neces- 
sary in order to prevent the increase of undesirable organisms that may 
be carried in from time to time. Hampshire? investigated the cause 
of a pitting, or liquefaction in spots, of the grain and flesh surfaces of 
sheep skins, a damage known to the trade as run pelts. He found that 
the pitting was caused by several species of wormlike organisms belong- 
ing to the family Nemathelminthes and growing to a length of about 
one millimeter. Apparently they are killed by simple drying. They 
were found in great numbers in the sweat chambers, but not on skins 
which had not yet entered the chambers. In laboratory experiments, 
they produced a pitting of the skin in the presence of a small amount 
of ammonia, such as is always present in the sweat chambers. It was 
found that uniform slipping of the wool could be produced by incubat- 
ing the skin in a clean vessel which excluded all organisms other than 
those present on the incoming skin, and skin treated in this way was 
free from pitting. It would seem that the danger of run pelts can be 
completely avoided by making certain of the cleanliness of the sweat 
chamber before the skins enter. 

Upon coming from the sweat chamber, the skins are usually put 


*The Mechanism of Unhairing. J. A. Wilson and Guido Daub. Presented before th 
Leather Division at the 64th meeting of the American Chemical Society. Publication ee 
photomicrographs reserved for this book. : 

* Causes of Run Pelts in the Sweating Process. P., Hampshire. J. Soc. Leather Trades 
Chem. 5 (1921), 20. 


UNHAIRING AND SCUDDING 155 


into saturated lime water and left there for a few hours or over night. 
Although this treatment is not essential and is sometimes omitted, it 
has the advantage of decreasing the danger of damage to the skins 
through putrefaction. The next step is the actual removal of the hair 
and epidermis. In modern practice, this is accomplished by means of 
an unhairing machine in which the skin is backed by a rubber slab and 
blunt knife blades pass over the hair side, under low pressure, rubbing 


Grams CaO Left per 100 Grams Dry Skin ~ 


1 2 3 4 5 6 ry 
Hours of Washing 


Fic. 68.—Removal of lime from unhaired skin by washing. 


off the hair and epidermis. Often the blades are set in rollers which 
rotate as they pass over the skin. 

The skin is then placed over a beam and scudded. The beam, from 
which the beamhouse derived its name, is a convex wooden slab sloping 
upward from the floor, at an angle of about 30°, to a point about three 
feet higher, which gives it a length of about six feet. The beamster, 
leaning over the beam, pushes a specially designed, two-handled knife 
over the skin downward and to left and right, forcing the remnants of 
the glands, lime soaps, dirt, and any remaining hairs out of the hair 
follicles and pores.. This operation is known as scudding. 

Goat skins can be scudded satisfactorily by machine after the bating 


150 THE CHEMISTRY OF LEATHER MANUFACTURE 


operation, but the author knows of no machine that can replace a good 
beamster for scudding calf skins after liming. Scudding can usually be 
done better by hand than by machine because the hair follicles slope in 
many different directions. If the knife stroke is made in the direction 
of the hair, from root to tip, the dirt in the follicles is easily squeezed 
out, whereas there is a tendency for-it to be trapped by a stroke in the 
opposite’ direction. There is a sufficient degree of transparency to a 
limed skin to enable the beamster to see the dirt and pigment in the 
follicles and he directs his knife first one way and then another until 
the skin appears clean. He is also on the lookout for fine hairs not 
removed by the machine. The bulb of a new hair is as deeply seated 
as that of an old one, but there may not be enough of the new hair 
protruding above the surface of the skin to be gripped by the knives 
of the unhairing machine. 

After the scudding operation, the skins are washed thoroughly to 
remove as much lime as possible. This washing is of considerable im- 
portance because any great excess of lime carried forward interferes 
with the later processes. It is customary to wash the skins in a revolv- 
ing drum through which fresh water is continually passing. Wood ® 
followed the removal of lime during washing and showed that little is 
to be gained by continuing the washing for more than two hours. The 
tendency, however, is to wash the skins for a shorter time than this 
and to take care of the residual lime by other means. Fig. 68 shows the 
extent of lime removal with time during a typical washing operation. 
The lime left in the skins appears to approach a limiting value, due to 
the lime which has carbonated as well as that in chemical combination 
with the skin. 


Liming. 


The commonest method in use today for effecting the separation of 
the epidermal system from the true skin is also one of ancient origin 
and is known as liming from the fact that saturated lime water is used. 
Formerly a lime liquor was prepared simply by filling a vat with water 
and adding calcium hydroxide greatly in excess of saturation. The 
skins, after soaking, were put into this liquor and allowed to remain 
there until the hair and epidermis had become so loosened that they 
could be rubbed off with very little pressure. Often the skins were 
removed each day and fresh lime added in order to hasten the action. 
But with a fresh lime liquor it usually required weeks for the skins to 
get into a state where the hair would slip easily. It was discovered that 
less time was required for each succeeding lot of skins passing through 
a given liquor. The longer a liquor was used the more it became 
charged with ammonia, other protein decomposition products, bacteria 
and enzymes, all of which assisted in loosening the hair, The older 
liquors, however, attacked the collagen fibers to a greater extent and also 
produced less swelling of the skin proteins than fresh liquors. 


* The Puering, Bating and Drenching of Skins, J, T, . 
London (1912). 8 J Wood. E. & F. N, Spon, 


ae ee 


UNHAIRING AND SCUDDING 157 


As more was learned of the action of lime liquors, it became cus- 
-tomary to employ a series of liquors for each lot of skins. The skins 
were put first into the oldest liquor in order to start the loosening of 
the hair. Each day they were moved into a fresher liquor and finally 
into one quite fresh. This system is still in use in some tanneries, but 
the modern tendency is toward quicker methods. 

When lime alone was used in making lime liquors, it usually re- 
quired from one to three weeks to cause the hair to slip easily, during 
which time a considerable amount of collagen became hydrolyzed, espe- 
cially in old liquors or in liquors not kept completely saturated with 
lime at all times. Bacteria are very sensitive to changes in pH value and 
many proteolytic bacteria present in lime liquors which are compara- 
tively inactive at a pH value of 12.5, that of an ordinary lime liquor, 
become very active as the pH value falls to lower values. In order 
to guard against the danger of incomplete saturation of the liquors 
with lime, mechanical agitators have been devised, one of the simplest 
being a paddle wheel set in the vat. By keeping the undissolved lime 
continually stirred up, the solution is kept almost at the saturation point. 

With increasing demand for speed of operation and conservation of 
the skin collagen, sharpening agents have come into wide use, the prin- 
cipal ones being arsenic sulfide, sodium sulfide, and sodium hydroxide. 
The judicious use of these materials, in conjunction with lime, has 
reduced the time required to unhair skins from weeks to as many days. 
More attention was paid also to temperature. In some of the old tan- 
neries not equipped to heat the liquors, a much longer time had to be 
allowed for unhairing in winter than in summer. It 1s now customary 
to maintain a uniform temperature of from 20° to 25° C. the year 
round. 

Arsenic disulfide was one of the first sharpening agents to be em- 
ployed. It was mixed with the lime before slaking in the proportion 
of about one part of sulfide to twenty-five parts of lime and from this 
mixture a liquor was made of such concentration that the hair would 
not be damaged, but would slip easily in two or three days. Sodium 
sulfide is now used more commonly than arsenic, being cheaper and 
somewhat more effective in loosening the hair. It is used at about 
0.01 molar concentration in a solution kept saturated with lime. 

The action of a lime liquor sharpened with sodium sulfide upon a 
calf skin is illustrated in Fig. 69. A fresh calf skin was put into a 
solution containing 0.7 gram of Na,S per liter and calcium hydroxide 
well in excess of saturation. The liquor was agitated frequently and 
kept at a temperature of 25°C. Strips of the skin were examined at 
intervals as in the study of the sweating process. The skin from the 
sweating process was in a soft, flaccid condition, while that from the 
lime liquor was plump and rubbery, but the fate of the epithelial cells 
of the Malpighian layer was the same in both cases. Sections of speci- 
mens taken at intervals showed these cells slowly disintegrating and 
leaving the corneous layer, hairs, and glands separated from the derma. 
Fig. 69 shows a section taken after the skin had been in the lime liquor 
for 48 hours. Part of the upper region of the section is shown at 


Fig. 69.—Vertical Section of Calf Skin. 
(After 48 hours in lime liquor.) 


Eyepiece: none. 
Thickness of section: 40 u. Objective: 32-mm. 


Stains: Weigert’s resorcin-fuchsin Wratten filters: B-green; E-orange. 


Location: butt. 


and picro-red. Magnification: 25 diameters, 


158 


i te he 


Fig. 70.—Vertical Section of Thermostat Layer of Calf Skin. 
(After 48 hours in lime liquor. ) 


Eyepiece: 5X. 

Objective: 16-mm. 

Wratten filters: B-green; E-orange, 
Magnification: 135 diameters, 


Location: butt. 

Thickness of section: 40 wu. 

Stains: Weigert’s resorcin-fuchsin 
and picro-red. 


159 


160° THE CHEMISTRY AOR ea roeh MANUFACTURE 


higher magnification in Fig. 70. The section is from the same skin 
as that shown in Fig. 18, which represents the fresh skin as it existed 
in life. 

The lime has completely destroyed the Malpighian layer of the 
epidermis and the corneous layer appears as a nearly continuous line 
somewhat separated from the true skin. The epithelial cells of the hair 
follicles have been completely broken up leaving the hair, with adhering 
patches of corneous layer, free to be swept out by the action of the 
unhairing machine. The sudoriferous glands have disintegrated, leav- 
ing empty spaces, and the sebaceous glands may be seen lodged in 
pockets opening into the hair follicles. The erector pili muscles are 
still intact and can be seen runing upward to the left from the region 
of the hair bulbs. In the thermostat layer, as well as in the deepest 
layer of the skin, the elastin fibers appear as fine, black threads. These 
fibers do not appear prominently in Fig. 18 because this section was 
stained with the object of showing greater detail in other parts. 

Although the hair loosening operation can be effected easily in a 
single liquor acting for two or three days, some tanners still prefer 
to use a series of liquors, claiming that they get a result better adapted 
for the particular kinds of leather they desire to make. They lessen 
the extra amount of labor involved in handling the skins by a system 
of reeling from vat to vat. The skins are all hooked or tied together, 
the head of one to the tail of another, and the whole lot is passed over 
a reel from one vat to another, the last skin in being the first to come 
out. The skins are put first into the oldest liquor and then reeled into 
a fresher liquor each day until ready to be unhaired. 


Plumping and Falling. 


When animal skin is immersed in dilute solutions of acid or alkali, 
the protein matter ‘swells by absorbing some of the solution, but the 
effect to a casual observer is not so much one of swelling as of increased 
resiliency of the skin, due to its fibrous structure. ‘The collagen fibers, 
in swelling, tend to fill up the interstices between them and the full 
increase in volume of the protein matter is not evident from the appear- 
ance of the skin. .A skin in which the fibers are not swollen may con-_ 
tain practically as much water as one whose fibers are swollen, as in 
lime water, but the bulk of the water in the first skin is held only loosely 
between the fibers and may be squeezed out by the application of slight 
pressure, whereas that in the second is present within the substance of 
the fibers, like the water absorbed by a solid block of gelatin jelly, and 
cannot be removed, except by the application of enormous forces. 
During the swelling of the protein matter, the tanner observes in the 
skin an increasing resistance to compression, to which he has given 
the name plumping, the term falling indicating the reverse action. 

Wood, Sand and Law ‘ devised an apparatus for determining when 

*The Quantitative Determination of the Falling of Skin in the Puering or Bating 


Process. J. T. Wood, Hi. J, Si Sand ‘and: D. J. Law, J, Soc. Chem. Ind. 31 (1912), 210 
and 32 (1913), 398. 


UNHAIRING AND SCUDDING 101 


a skin had become completely fallen during the bating process which 
consisted of a sensitive thickness gauge in which the pressure exerted 
upon I square centimeter of skin could be varied by means of weights. 
The point of complete falling of a skin was taken as that at which no 
recovery in thickness of the skin took place upon removing the weights. 
The apparatus was also used to measure the apparent modulus of elas- 
ticity of the skin and this was considered to be a measure of the degree 
of plumping. 

This method suggested to Wilson and Gallun® another which is 
more suitable for certain purposes. Their apparatus consisted of a 
Randall and Stickney thickness gauge ** with a flat, metal base upon 
which a small piece of skin could be placed, and a plunger, having a 
circular base I square centimeter in area, capable of pressing on the 
surface of the skin under constant pressure. The apparent thickness 
of the skin, as shown on the dial of the instrument, being determined 
by the position of the plunger, decreased with time as the plunger caused 
an increasing degree of compression. For this reason and in order to 
get comparative readings, all gauge readings were taken a fixed length 
of time after dropping the plunger onto the skin. In order to measure 
the degree of plumping of skin in a given liquor under fixed conditions, 
they first measured the resistance to compression of a small piece of 
skin under standard conditions. This same piece of skin was then 
subjected to the conditions of the test and its resistance to compression 
measured again. In each case the gauge reading was taken as a measure 
of the resistance to compression. The ratio of the final to the initial 
gauge reading is a measure of the degree of plumping of the skin. 
Their measurements of the degree of plumping of calf skin as a func- 
tion of pH value are given in Chapter 9. 

If a skin in the alkaline state is plumped or swollen excessively, it 
suffers permanent distortion and the value of the final leather is lowered. 
Some knowledge of the degree of plumping of skin in liquors used for 
unhairing is therefore much to be desired. 

Atkin ® was able to reason from the work of Procter, Wilson, and 
Loeb, which was discussed in Chapter 5 in connection with the swelling 
of protein jellies, that arsenic disulfide is preferable to sodium sulfide 
for certain kinds of skin where fineness of grain surface is of para- 
mount importance in the finished leather. Loeb showed that diacid 
bases produce a maximum swelling of gelatin jelly only half as great 
as that produced by monacid bases. Atkin confirmed this for the 
swelling of hide powder and showed that the weak base ammonium 
hydroxide produces as much swelling as sodium hydroxide at the same 
pH values. When arsenic disulfide is slaked with lime and used in a 
fresh liquor, the solute consists only of calcium hydroxide, calcium 
sulfhydrate, and calcium sulfarsenite. But when sodium sulfide is used 
as the sharpening agent for a lime liquor, sodium hydroxide and -sodium 


5 Direct Determination of the Plumping Power of Tan Liquors. J. A. Wilson and A. 
F, Gallun, Jr. Ind. Eng, Chem. 15 (1923), 376. 

@ Made by Randall and Stickney, Waltham, Mass. 

*Notes on the Chemistry of Liine Liquors Used in the Tannery, W, R, Atkin. 


7 


J. Ind. Eng. Chem. 14 (1922), 412, 


162 THE CHEMISTRY OF LEATHER MANUFACTURE 


sulfhydrate are present. It would therefore be expected that the use 
of sodium sulfide would result in a greater plumping of the skin than 
the use of arsenic sulfide, which gives a liquor containing only divalent 
cations. In actual practice, when arsenic sulfide is used to sharpen lime 
liquors for the unhairing of goat skins in the manufacture of glazed kid 
leather, the final leather has a smoother and silkier grain surface than 
when sodium sulfide is used in the lime liquors. 

It might be inferred from this that it is preferable to use arsenic 
sulfide for all kinds of skin where smoothness of grain is desired, but 
this is not necessarily so. All skins are not equally sensitive to injury 
through plumping. What may prove to be excessive plumping for goat 
skins may not have any deleterious effect at all on a calf skin and one 
type of calf skin might be more resistant to permanent distortion than 
another. The greater speed of action and lower cost of sodium sulfide 
makes its use preferable in all cases where it does no harm to the skins. 

It sometimes happens that a skin can be unhaired less readily the 
more it is plumped. This seems to be due to the overlapping scales of 
the hair, which open upward as shown in Fig. 6. When the skin is put 
into a liquor in which it swells considerably, the hair becomes tightly 
pinched by the skin and at the same time the scales become distended, 
their ends wedging themselves into the sides of the follicles in such 
manner as to resist any attempt to pull the hair out. If the fine hairs 
are not removed from a skin while it is still in the alkaline condition, 
but are allowed to remain in place until after the tanning operation, they 
again become firmly fixed in place, apparently because of the distention 
of the hair scales and the permanent plumping of the skin produced by 
the tannage. 


Fresh vs. Mellow Lime Liquors. 


A much used lime liquor, charged with decomposition products of 
the skin, bacteria and enzymes, is usually referred to as mellow. Where 
unsharpened lime liquors are used, a mellow liquor causes a much more 
rapid loosening of the hair and much less plumping of the skin than a 
fresh liquor. This difference is not due to any difference in hydroxide- 
ion concentration for Wood and Law? have shown that a mellow lime 
liquor has a pH value practically the same as that of pure saturated lime 
water. They found also that the pH value is but little affected by the 
addition of small quantities of sodium sulfide and this has been con- 
firmed in the author’s laboratories. The decrease in plumping power 
of a lime liquor with use may be ascribed to the calcium salts formed, 
which tend to repress the swelling of proteins by calcium hydroxide. 
But the increasing power to loosen the hair must be attributed to the 
protein decomposition products, bacteria, enzymes, or the lesser swell- 
ing of the skin at the same pH value, or possibly to a combination of 
all four factors. | 

Wood and Law regard the growth of bacteria in lime liquors as 
the principal factor in the production of mellowness. They examined 


7 Light Leather Liming Control. J. T. Wood and D. J. Law. Collegium (1912), 121. 


UNHAIRING AND SCUDDING 163 


an old lime liquor in which skins had been worked for 3 to 4 weeks and 
obtained a count of 50,000 bacteria per cubic centimeter of a type 
capable of developing in ordinary nutrient gelatin containing ammonia. 
They identified Micrococcus flavus liquefaciens and B. prodigiosus, 
both of which are known to produce proteolytic enzymes. The bac- 
teria found on the roots of wool from the sweating process were found 
to be capable of growing in a liquid as alkaline as 0.05 normal. These 
appear to be similar to the bacteria commonly present in mellow 
lime liquors and Wood considers it highly probable that the unhair- 
ing action both in the sweat chamber and in mellow lime liquors 
is due to the same bacteria, not necessarily belonging to a single 
species. 

Stiasny * also showed that bacteria play an important role in old 
lime liquors. An untreated mellow lime liquor caused a loosening of 
the hair of calf skin in 24 hours, but in a test where chloroform was 
added to the same liquor to check bacterial action the liquor was not 
able to cause any loosening of the hair in 3 days. A portion of the 
untreated liquor was freed from ammonia by heating to 60° C. and 
passing carbon dioxide-free air through it for 4 hours. It then showed 
an unhairing power as great as before, but a lesser solvent action on 
the hide substance, indicating that the unhairing action is due to bac- 
terial action rather than to the ammonia ordinarily present in mellow 
liquors. 

Since sterile lime water appears to have but little unhairing action 
on skins, it was long thought that bacteria were necessary for this 
action, where no sharpening agent was employed. But Schlichte ® 
found that skin previously sterilized by the Seymour-Jones process, 
with mercuric chloride and formic acid, could be unhaired easily after 
two weeks of contact with saturated lime water under sterile conditions. 
Wood and Law," however, pointed out that the action may have been 
influenced by the previous swelling of the skin in the sterilizing solu- 
tion. This is intelligible from the viewpoint of Stiasny,‘ who regards 
proteins as peptones held together relatively loosely by means of 
secondary valency forces. The peptones are considered to be built 
up of peptides held together by forces of primary valence. He as- 
-sumes that the swelling of a protein jelly causes a diminution in the 
forces holding the peptones together. On this basis, the swollen protein, 
or one in which the bonds between the peptones had been weakened 
through previous swelling, would be attacked by hydrolyzing agents 
much more readily than the unswollen protein. In support of this 
view, he finds that collagen is attacked by trypsin very much more 
rapidly when swollen by potassium thiocyanate or iodide solutions and 
that the action then goes only to the peptone stage. 

It was suggested by the author * that barium and calcium hydroxides 


® The Nature of the Liming Process. E. Stiasny. Gerber (1906); English translation, 
J. Soc. Leather Trades Chem. 3 (1919), 129. 

®A Study of the Changes in Skins during Their Conversion into Leather, A. A. 
Schlichte. J. Am. Leather Chem. Assoc. 10 (1915), 526 and 585. 

7 Note on the Action of Lime in the Unhairing Process. J. T. Wood and D, J. Law. 
Peer, Chen. Ind. 35 (1916), 58s. 

™ Some Modern Problems in Leather Chemistry. E. Stiasny. Science 57 (1923), 483. 
( Alia of Leather Chemistry. J. A. Wilson. J. Am. Leather Chem. Assoc. 12 
1917), 108, 


164 THE CHEMISTRY OF LEATHER MANUFACIGRe 


hydrolyze proteins to a lesser extent than the hydroxides of sodium 
or ammonium because of the higher valency of the cations. The swell- 
ing of proteins in alkaline solution is due to the pull of the cations of 
the protein salt, which tend to diffuse from the region of high con- 
centration of ions in the jelly to the region of lower concentration 
in the surrounding solution. If this pull is sufficiently great, we might 
reasonably expect a breaking up of the units making up the protein 
jelly. A sodium or ammonium ion exerts its entire pull upon a single 
unit, whereas the pull of a divalent cation is divided between two units, 
making the tendency towards decomposing the protein only half as 
great. This valency effect, however, is not the only one playing a 
part in sterile unhairing liquors because the mere replacement of half 
of the hydroxide ions of lime water by sulfhydrate ions is sufficient 
to cause a very marked increase in the rate of unhairing. Wood and 
Law suggested that Schlichte’s observation of the unhairing power of 
sterile lime water is further complicated by the formation of sulfur 
compounds by the action of lime on the easily dissolved sulfur of the 
hair. Such compounds are capable of loosening the hair. 


Unhairing by Means of Other Alkalies. 


Pure solutions of sodium hydroxide and sodium sulfide quickly 
destroy the hair and epidermis when sufficiently concentrated. A 
2-per cent solution of Na,S at 25°C. will dissolve the hair and epi- 
dermis from the surface of a calf skin in about 2 hours, during which 
time only a comparatively small amount of collagen is destroyed. This 
treatment has been applied with considerable success to heavy hides, 
especially those which had previously been dried, and was a great help 
in speeding up the production of army leathers during the war. The 
hides were put into the sulfide solution, which was agitated by means 
of a paddle wheel. After several hours the hides were transferred 
to a solution of sodium bicarbonate or calcium chloride in order to 
stop the caustic action of the sodium sulfide. They were then washed 
and were ready for bating or tanning. The hair was completely dis- 
solved from the surface of the hides in the sulfide liquor, but the 
action was so rapid that they had to be removed before the sulfide 
had diffused into them to the depth of the hair bulbs. As a result, 
the hair bulbs were usually left in the hides intact, as could be shown 
by examining sections under the microscope, but this apparently did 
not lower the value of the leather in any way. 

With this method of unhairing, it was found economical to use 
the same liquor for a number of consecutive lots of skins, adding just 
enough fresh sodium sulfide each time to maintain the necessary con- 
centration. The liquors soon became heavily charged with protein 
decomposition products which are soluble in alkaline solution, but 
are precipitated by rendering the solution faintly acid. Kadish and 


UNHAIRING AND SCUDDING 165 


Kadish ** made use of this fact in a scheme for recovering this nitrog- 
enous matter as fertilizer. The waste liquors were run into a mixing 
chamber where they were reacted upon by sulfuric, sulfurous, or 
other acid. The precipitated nitrogenous matter was separated from 
the mother liquor and the hydrogen sulfide was recovered separately 
in such manner as to make the entire operation continuous. 

Using sodium hydroxide instead of the sulfide, a similar unhairing 
action is obtained, but the skin becomes much more swollen and 
plumped. For the finer grades of light skins, where a smooth grain 
surface is required, neither sodium hydroxide nor sulfide solutions 
can be used alone because of the rough grain resulting from the 
excessive plumping. 

It is not an uncommon practice in dewooling sheep skins to paint 
them on the flesh side with a paste made of a mixture of lime and 
sodium sulfide. The skins are then folded, wool side out, and left 
until the sulfide has diffused into the skins as far as the hair bulbs. 
When these are destroyed, the wool can be pulled or brushed out. As 
a rule, the skins are thrown over a beam and the wool is worked off 
by a beamster. The skins are then limed, washed, bated, and pickled, 
in which condition they may be kept until required for tanning. 
Sometimes the paste is made from lime and arsenic sulfide. 

Solutions of ammonia in twice-molar concentration have a very 
marked unhairing action on fresh skins. The author found that fresh 
calf skins could be unhaired quite satisfactorily after only two hours’ 
immersion in such a solution. The skin swells but very little and 
the grain surface is left remarkably smooth and silky. If the skin 
is left in the solution longer than is necessary, however, there 1s 
danger of it suffering damage because of the powerful action of the 
ammonia on the collagen fibers. Since the unhairing’ powers of 
ammonia have long been known, it has often been wondered why 
its use has not become widespread. In an investigation, the author 
found that it could not be relied upon for unhairing the ordinary run 
of skins in commerce because its action is influenced by the previous 
treatment of the skin. On some skins, the ammonia would loosen 
the hair only in patches. In one experiment, a piece of fresh calf 
skin was cut into two pieces. One was put directly into twice-molar 
ammonia solution and the hair was loosened quite satisfactorily in two 
hours. The other was soaked in molar acetic acid for an hour, washed, 
neutralized with ammonia, and then put into the twice-normal ammonia 
solution. But there was no appreciable loosening of the hair after 
several hours. 

Stiasny “* studied the effect of adding different salts upon the un- 
hairing action of ammonia. He used a series of liquors each consisting 
of half-normal ammonia and 0.07 normal chloride of sodium, calcium, 
barium, or zinc. One liquor contained ammonia alone. A piece of 
fresh calf skin was put into each. After 2 days the piece in am- 


VY, H. Kadish, U. S. patent 1,269,189 (1918); V. H. and H. L. Kadish, U. S. patent 
1,298,960 (1919). 
144The Nature of the Liming Process, loc. cit. 


106 THE CHEMISTRY OF LEATHER MANUFACTURE 


monia alone had increased in weight 65.5 per cent, the one in the 
solution containing sodium chloride 45.8 per cent, in calcium chloride 
14.9 per cent, in barium chloride 19.8 per cent, and in the solution 
containing zinc chloride 31.4 per cent. The hair was loosened in 
the solution of ammonia alone and in the one containing sodium 
chloride, but not in the others. Atkin’ has pointed out that the dif- 
ference in repression: of swelling by the different salts may be at- 
tributed to the valency of the cation. It is, of course, evident that 
the difference in unhairing action may be explained in the same way. 
Stiasny, however, looked upon the difference in action as due to the 
formation of complexes between the ammonia and the divalent cations, 
giving salts of the type Ca(NH,),Ch. 


Unhairing by Means of Acids. 


In 1916, Mr. J. T. Wood sent the author a piece of calf skin 
which had been sterilized by the Seymour-Jones process. The formic 
acid had caused a loosening of the hair, which Mr. Wood says was 
marked in 8 days. Thuau*® and Nihoul !’ had previously shown that 
sulfurous acid will cause a loosening of the hair of skins, if used in 
solutions that will prevent the swelling of the skin, as in the presence 
of salt. Marriott’® found that salted hide could be unhaired by 
immersion in 0.25-per cent acetic acid solution for Q days. 

In no case was the hair loosening by means of acid as satisfactory 
as can be obtained in alkaline solution. The acid seems to attack only 
the deepest layer of the epithelial cells of the Malpighian layer, leav- 
ing most of the epidermis intact, to be removed with the hair. It seems 
doubtful that acid will ever replace alkaline solutions for unhairing. 


Unhairing by Means of Pancreatin. 


In 1913, Rohm ** described a process for unhairing and bating skins 
in one operation, involving the use of an alkaline solution of pan- 
creatin. Since then pancreatin has often been listed as an unhairing 
agent. In 1920, Hollander *? described Réhm’s process as having a 
number of advantages over the old system of liming and claimed that 
it depends entirely upon enzyme action for unhairing. According to 
his description, the skins ‘are first soaked for 1 day in dilute sodium 
‘hydroxide solution and then transferred to a dilute solution of sodium 
bicarbonate to which the enzyme is added after the swelling due to 
the alkali has been counteracted. Twenty-four hours later the hair is 
completely loosened and can be rubbed off. 

Wilson and Gallun ?! investigated this method with the object of 


© Notes on the Chemistry of Lime Liquors Used in the Tannery, Joc. cit. 

16 Unhairing with Sulfurous Acid. U. J. Thuau. Collegium (1908), 362. 

% Unhairing with Sulfurous Acid. E. Nihoul. Bourse aux Cuirs de Lidge (1908), 8. 

18 Acid Unhairing. R, H. Marriott. J. Soc. Leather. Trades Chem. 5 (1921), 2. 

12 A New System of Liming. O. Réhm. Collegium (1913), 374; J. Am. Leather Chem, 
Assoc. 8 (1913), 408. : 

7° Unhairing Hides and Skins by Enzyme Action. C. S. Hollander, J. Am. Leather 
Chem. Assoc, 15 (1920), 477. 

» Pancreatin as an Unhairing Agent. J. A. Wilson and A. F, Gallun, Jr. Ind. Eng. 
Chem. 15 (1923), 267. 


_UNHAIRING AND SCUDDING 167 


determining the specific rdle played by the enzyme. They made a pre- 
liminary examination by soaking pieces of thoroughly cleansed calf 
skin in 0.05 molar sodium hydroxide solution for 1 day, replacing the 
solution next day by 0.1 molar sodium bicarbonate solution, and 5 
hours later transferring the pieces to a solution made by diluting 18 
cubic centimeters of molar sodium hydroxide, 2.8 grams of monosodium 
phosphate, and 1 gram of U.S.P. pancreatin to 1 liter. The pH value 
of the solution was found to be 7.52 at 25° C., lying well within the 
range of optimum activity of this enzyme. Two experiments were 
run at a temperature of 25° C., but in one the solutions were left ex- 
posed to air, as would be the case in practice, while in the other they 
were covered with a layer of toluene to check bacterial action. After 
the pieces had been in the enzyme solutions for 24 hours, the hair 
of the pieces from the solutions exposed to air could be rubbed off 
with the greatest ease, leaving the erain surface clean and white, 
but that of the pieces from the solutions under toluene remained 
firmly fixed. This seemed to indicate that the unhairing action obtained 
at 25° was not due to the enzyme, but probably to proteolytic bacteria 
or their products. 

Because of the doubt thus cast upon the role played by pancreatin 
sn this method of unhairing, Wilson and Gallun carried the investiga- 
tion further, paying particular attention to the action of pancreatin at 
4o° C., the temperature of its maximum activity. The studies were 
made upon pieces of fresh calf skin, about 5 x3 inches, which had 
been thoroughly soaked and cleansed. Each experiment was carried 
out both at 25° and at 4o°C. The action of the enzyme solution 
upon the skin in each test was compared with the action of a blank 
identical with the enzyme solution except for the fact that it con- 
tained no enzyme. This solution was prepared by diluting 18 cubic 
centimeters of molar sodium hydroxide solution and 2.8 grams of 
monosodium phosphate to 1 liter and all enzyme solutions were made 
by adding to it 1 gram of pancreatin per liter. The pH values did 
not vary more than o.1 from the value 7.6 in any case. The enzyme 
solutions and blanks as well as solutions used for the pretreatment 
of the skin were all covered with a layer of toluene to check bacterial 
‘action. The results were checked on separate occasions with pieces 
of skin from different sources. 

The effect of pancreatin upon skin not previously soaked in sodium 
hydroxide solution, or any other swelling agent, was studied first. 
After 24 hours of contact of skin and solution, little action was notice- 
able either at 25° or 40°, but after 48 hours the collagen fibers of 
the skin in the enzyme solution at 40° began to dissolve very rapidly, 
the action proceeding from the flesh side, but there was no indication © 
of the hair becoming loosened. On the other hand, the skin in the 
blank at 40° and those at 25° in both blank and enzyme solution still 
remained but little affected. It was evident that pancreatin has a 
more powerful solvent action upon the collagen fibers than upon the 
epidermis of a skin not previously swollen with acid or alkali. The 
time factor involved in the destruction of the collagen fibers is in- 


168 THE CHEMISTRY OF LEATHER MANUFACTURE 


teresting. The action seemed to indicate that the fibers were coated 
with some material more resistant to tryptic digestion than the col- 
lagen beneath it. Possibly this supposed covering may be found to 
bear some relation to what Seymour-Jones 7? has called the fiber 
“sarcolemma.”’ | 

In the next series of experiments, the pieces of skin were kept 
for 24 hours in 0.05 molar sodium hydroxide solution at 25° and 40° C., 
respectively. The solutions were then replaced by 0.1 molar sodium 
bicarbonate solutions of corresponding temperatures, and 5 hours 
later by the enzyme and blank solutions, in which the skins remained 
for 24 hours. The unhairing action in the enzyme solution at 40° 
was completely satisfactory, indicating that, at this temperature, pan- 
creatin may be considered an unhairing agent for calf skin previously 
swollen in dilute sodium hydroxide solution. A very slight unhairing 
action was noticeable in the blank at 40°, evidently due to the previous 
treatment with alkali. No unhairing action could be detected in the 
blank or enzyme solution at 25°. 

The preceding series of experiments was then repeated exactly, 
except that 0.05 molar hydrochloric acid solution was substituted for 
the alkali as the swelling agent. At 25° there was no visible un- 
hairing action either in the blank or enzyme solution. In the hydro- 
chloric acid solutions in the bath at 40°, the pieces of skin began 
to jelly; there was no further change in the piece transferred to 
the blank at 40°, but the piece put into the enzyme solution at 40° 
was quickly destroyed, the collagen passing into solution, leaving the 
epidermis and hair floating in the liquor. The opposite effects of acid 
and alkali upon the skin at 40° is interesting. 0.05 molar sodium 
hydroxide solution hydrolyzes the epidermis more rapidly than the 
collagen fibers, whereas 0.05 molar hydrochloric acid hydrolyzes collagen 
much more rapidly than it does the epidermis. 

The experiment was repeated except for the fact that the pre- 
treatment with hydrochloric acid was done at 25° and the digestion 
with pancreatin at 40°. After the skin had been in the pancreatin 
solution for 24 hours, the hair was completely loosened, showing that 
the effectiveness of pancreatin as an unhairing agent depends upon the 
previous swelling of the skin, but regardless of whether the swell- 
ing is caused by acid or alkali. The fact that pretreatment with sodium 
hydroxide in the experiment with alkalies was done at 40° did not 
seriously influence the result for, when another piece of skin was 
soaked in 0.05 molar sodium hydroxide solution at 25° for a day and 
then in the pancreatin solution at 40°, the unhairing action was entirely 
satisfactory. 

Experiments dealing with the action of pancreatin upon skins 
previously treated with ammonia were carried out exactly like those 


of the sodium hydroxide series, except for the replacement of the. 
0.05 molar sodium hydroxide solution by 0.50 molar ammonium. 


hydroxide solution. The hair was loosened to some extent by the 


( Bye ey of the Skin. Alfred Seymour-Jones. J. Soc. Ecotier Trades Chem. 2 
1916), 203. d : 


a ee tls ok 


UNHAIRING AND SCUDDING 169 


pretreatment with ammonia, more at 40° than at 25°. After the pieces 
had been in the blank and enzyme solutions for 24 hours, they all 
showed some unhairing action, but in no case was it entirely satisfac- 
tory. The degree of action might be given a very rough rating by 
calling that in the enzyme solution at 40° 75 per cent, that in the blank 
at 40° 50 per cent, and that in both blank and enzyme solutions at 25° 
25 per cent. Evidently the pretreatment of skin with ammonia, which 
is itself an unhairing agent, does not assist the unhairing action of 
pancreatin nearly so much as pretreatment with materials whose action 
is primarily to swell the skin. 


Combined Bating and Unhairing by Means of Pancreatin. 


Wilson and Gallun extended their investigation to an examination 
of the effect of the pancreatin upon the elastin fibers of the skin, 
the work of Wilson and Daub having indicated previously that the 
fundamental action of bating is the removal of elastin fibers from 
the skin. The work of Wilson and Daub will be described in the next 
chapter. Pieces of skin were taken from the various experiments 
after the pancreatin had acted upon them. These were imbedded, 
sectioned, stained, and mounted for examination, as described in 
Chapter 2. 

When the pancreatin method of unhairing is used in practice, the 
liquors are left exposed to air. The experiments of Wilson and Gallun 
show that the hair loosening can then be effected at a temperature 
of 25°C., but that the action is apparently not due to enzyme, but 
rather to bacteria, since it is checked by covering the solutions with 
toluene. But, if pancreatin is not the active agent, we should expect 
the action not to be accompanied by elastin removal. Fig. 71 cor- 
roborates this view; where the hair loosening was effected by a pan- 
creatin solution at 25°, exposed to air, the epidermis is disintegrated 
and the hair loosened, but the elastin fibers remain undissolved and 
show in the upper half of the picture as fine, black threads running 
nearly horizontally. 

In the unhairing experiments where the skin from the enzyme 
solutions at 40° C. had not previously been swollen with acid or alkali, 
microscopic examination showed that all of the elastin had been dis- 
solved away from the flesh side of the skin in 24 hours, but none from 
the region just under the epidermis. The hard corneous layer of the 
epidermis had apparently acted as a membrane impermeable to the 
enzyme. In the ordinary methods of unhairing, such as liming, the 
unhairing agent acts upon the cells of the Malpighian layer, which 
lie between the corneous layer and the derma. The impermeability of 
the corneous layer to the enzyme explains why the pancreatin did not 
attack the Malpighian layer and loosen the hair. In acid or alkaline 
solutions, the corneous layer swells considerably and is thereby ren- 
dered more permeable. It is also attacked by the enzyme, when in 
the swollen condition, as shown by the fact that no corneous layer 
could be found in the sections examined. 


Fig. 71.—Vertical Section of Thermostat Layer of Calf Skin. 
(After 1 day in 0.1-per cent pancreatin solution at 25° C.) 


Location: butt. Eyepiece: 5X. 

Thickness of section: 30 pw. Objective: 8-mm. 

Stains: Van MHeurck’s logwood, Wratten filter: H-blue green. 
Daub’s bismarck brown. Magnification: 170 diameters, 


170 


Fig. 72.—Vertical Section of Thermostat Layer of Calf Skin. 
(After 1 day in 0.1I-per cent pancreatin solution at 4o° C.) 


Location: butt. Eyepiece: 5X. 

Thickness of section: 30 wp. Objective: 8-mm. 

Stains: Van MHeurck’s logwood, Wratten filter: H-blue green. 
Daub’s bismarck brown. Magnification: 170 diameters. 


171 


172 THE CHEMISTRY OF LEATHER MANUFACTURE 


Fig. 72 shows a section of calf skin which had been soaked in 
sodium hydroxide solution previous to digestion with pancreatin at 
40° C., under toluene. Not only is the epidermis destroyed and the 
hair loosened, but the skin is completely bated, as shown by the 
absence of elastin fibers. 

An interesting attempt to unhair skins by means of enzymes 
naturally occurring in the skin is that of H. C. Ross.** A 1-per cent 
solution of ammonium hydroxide is used to inactivate the foreign 
enzymes, while the thrombase found in the skins is activated by the 
addition of calcium lactate or polysulfide. It is mentioned that the 
thrombase may be assisted by the addition of trypsin or other pro- 
teolytic enzymes which will work in an alkaline medium. The unhair- 
ing is effected without destroying the epidermis, so that large sections 
thereof can be removed with the hair attached. Subsequent bating 
is unnecessary. In preparing dressing leathers, the solutions are heated, 
while for sole leathers cold liquids are employed, these allowing 
plumpifg to take place to a greater extent. How nearly the actual 
mechanism of this method of unhairing is suggested by the descrip- 
tion of the patent is open to question, but it would be interesting — 
to see a study made of it along lines similar to those of the experiments 
of Wilson and Gallun. 

Skins prepared for unhairing and scudding by means of pancreatin 
solutions are unhaired on a machine, scudded on the beam, and then 
washed, after which they are ready for tanning without further treat- 
ment. Skins from lime liquors are unhaired, scudded, washed and 
then either bated, delimed, drenched, or pickled before tanning. Some 
tanners put the skins directly into old vegetable tan liquors with- 
out giving them one of these treatments, but the tan liquor then becomes 
a deliming agent and has little value other than that of removing 
lime. 

Apparently anything that will hydrolyze the newly formed cells 
of the epidermis without injuring the rest of the skin is a satisfactory 
unhairing agent. Lime owes its popularity to the safety attending 
its use. Its limited solubility makes it possible to maintain a con- 
stant hydroxide-ion concentration at about 0.03 mole per liter simply 
by using an excess. This concentration is high enough to retard putre- 
faction considerably and yet not great enough to injure the skin 
itself, since the solute is a diacid base. It is entirely possible, however, 
that the popularity of lime will wane when some of the newer methods 
of unhairing reach a higher stage of development. 


*8 British Pat. 169,730, March 25, 1920. Chemical Abstracts 16 (1922), 853. 


Chapter 9. 
Bating. 


Perhaps the most curious of all the processes involved in making 
leather is that of bating. Little is known of its origin because it 
WaSea secret process, but: it is at least some centuries old. After 
the skins are taken from the lime liquors, unhaired, scudded, and 
washed, they still contain lime in the form of carbonate and in com- 
bination with the skin proteins. At this stage they are plump and 
rubbery and tanners have experienced many difficulties due to putting 
the stock directly into certain types of vegetable tan liquors when it 
was in this condition. The object of bating is to prepare the un- 
haired skins for tanning and originally consisted in keeping. them 
in a warm infusion of the dung of dogs or fowls until all plumpness 
had disappeared and the skins had become so soft as to retain the 
impression of thumb and finger when pinched and sufficiently porous 
to permit the passage of air under pressure. When hen or pigeon 
manure was used, the process was called bating, and when dog dung 
was used, it was called puering, but the term bating is now applied 
to the process generally, regardless of the materials used. The 
difference in terminology naturally disappeared with the advent of 
artificial bating materials. 

A common method for treating light skins was to put them into 
a vat filled with a liquor containing about Io0 grams of dog dung 
per liter, kept at a temperature of 40° C. by means of steam. A 
paddle wheel kept the liquor and skins in motion. During the action, 
the skins gradually lost the plumpness acquired in the lime liquors 
and became soft and raggy. The completion of the process was de- 
termined by the attainment of a certain degree of flaccidity, which the 
workmen could judge only after long experience. Hen or pigeon 
manure was sometimes used for light skins, but was more commonly 
applied to heavy hides because it penetrates more rapidly than dog 
dung, due apparently to the fact that it contains also the urinary 
products, especially urea. 

For many years this remained one of the mysterious processes 
of the tannery. It gave some tanners an improved product, which 
they could get in no other way known to them. But during the past 
thirty years there has been a persistent effort to determine the es- 
sential reactions of bating so that it might be carried out more reliably 
and with less offensive materials, or that it might be done away with 


173 


174 THE CHEMISTRY OF LEATHER MANUFACTURE 


entirely by treating the skins differently at other stages. For ex- 
ample, it had been suggested that the only important function of the 
bate is the removal of the insoluble lime compounds from the skin 
before tanning. But this was contested by those who believed that 
merely removing the lime was not sufficient. They regarded bating 
as a process necessary for the removal of certain undesirable protein 
constituents of the skin. In order to settle this question, investigators 
have made extensive studies of dungs, and of the skins and liquors, 
both before and after the process. - 

The greatest pioneer work in this field has been carried out by 
J. T. Wood, whose investigations, coupled with practical developments 
by O. Rohm and others, have led to the almost complete replacement 
of the obnoxious dungs by pancreatic enzymes. In his book, Wood ? 
says: ‘When learning the trade as an apprentice every fault in the 
leather was attributed to this part of the work, and the troubles and 
miseries of the ‘puer shop’ first caused me to take up the study of 
puering. I was determined to know the causes underlying the process. 
Puering is not only a filthy and disgusting operation, but is prejudicial 
to health, and in the nature of it is attended by more worry and 
trouble than all the rest of the processes in leather making put 
together.” 

Wood found the mineral matter of dungs to consist chiefly of 
the sulfates, chlorides, carbonates, and phosphates of sodium, potas- 
sium, ammonium, and calcium, and some silica. The most important 
organic constituents seemed to be the bacteria, enzymes, cellulose ma- 
terials, and fats. He found both peptic and tryptic enzymes, a rennin, 
an amylolytic enzyme, and a lipase. Since the bate liquor is usually 
faintly alkaline, it seemed likely that trypsin was active in the process 
and it was later shown that this enzyme does produce some of the 
effects of dung upon the skin. Wood also isolated from dog dung a 
species of B. coli which was found to yield an enzyme capable of 
acting upon the skin like trypsin. 

Artificial bates are now to be found upon the market which con- 
tain pancreatin, ammonium chloride, and supposedly inert fillers and 
these have largely supplanted the dung bates formerly used. But ma- 
terials other than those containing tryptic enzymes have also appeared 
on the market, as bates, to revive the old question as to the fundamental 
object to be attained by bating. These materials apparently give sat- 
isfactory results for some kinds of leather, even though some of 
them consist merely of carbohydrates, which yield organic acids by 
fermentation. The dung bates evidently had several different func- 
tions, but apparently all manufacturers of artificial bating materials 
did not concentrate their attentions upon the same functions. Numbers 
of preparations of quite different properties are sold as bating ma- 
terials and this has served to aggravate the confusion as to what 
constitutes a bating material. The several purposes served by these 
materials will be considered separately. 


, es Puering, Bating and Drenching of Skins. J. T. Wood. E. & F.N. Spon, London 
1912). 


BATING 178 


Falling. 


The one property which all of the various types of bating materials 
have in common is that of reducing the degree of swelling of the 
protein constituents of the limed skin, which action is known to the 
trade as falling. Indeed it would have been practically impossible 
for any artificial preparation to pass as a bate that did not have 
this property, because the degree of flaccidity of the skin was the 
accepted measure of the nearness to completion of the bating process. 

It will be apparent from the discussion of the swelling of protein 
jellies given in Chapter 5 that the degree of falling of a skin 
must be a function of hydrogen-ion concentration and also of the 
concentration of neutral salts. 

Wilson and Gallun 2 measured the degree of plumping of calf skin 
as a function of pH value by means of their method, which is described 
in Chapter 8. Pieces of unhaired skin, each about 2 centimeters square, 
were cut from the butt of a calf skin so as to insure the greatest 
degree of uniformity of structure. These were freed from lime by 
washing in a 1I2-per cent solution of sodium chloride containing a 
small amount of hydrochloric acid, and then neutralized in cold, sat- 
urated sodium bicarbonate solution. They were then washed and 
bated by keeping at 40° C. for 24 hours in a solution containing 0.1 
gram of U.S.P. pancreatin, 2.8 grams of monosodium phosphate, and 
18 cubic centimeters of molar sodium hydroxide solution per liter, 
giving a pH value of 7.7. Microscopic examination showed that this 
procedure removed all of the elastin fibers. The pieces were then 
washed in cold, running tap water, having a pH value of 8, for 24 
hours. They were then kept in distilled water in the refrigerator at 
7° C. until used for the tests. The condition in which the skin existed 
in this state was taken as a standard, as it was found to be easily 
reproducible. 

A series of 24 large reservoirs of test solutions was prepared, 
each having a final concentration of tenth-molar phosphoric acid plus 
the amount of sodium hydroxide required to give the desired pH value 
as determined by the hydrogen electrode. A range of pH values 
from 4 to II was covered. 

In each test a piece of skin in standard condition was placed 
in the Randall and Stickney thickness gauge described in Chapter 8. 
The gauge reading in every case was taken exactly five minutes after 
dropping the plunger onto the piece of skin. This was called the 
initial gauge reading. The skin was then shaken with water to bring 
it back to its natural shape and then put into 200 cubic centimeters 
of standard buffer solution of the desired pH value and kept in a 
thermostat refrigerator at 7° C. so as to reduce to a minimum any 
tendency towards putrefaction. After 24 hours, each solution was 
replaced by fresh buffer solution. After 4 days more, there being 


2The Points of Minimum Plumping of Calf Skin. J. A. Wilson and A. F, Gallun, Jr, 
Ind. Eng. Chem. 15 (1923), 71. 


170. THE CHEMISTRY OF LEATHER MANUFAC 


practically no change taking place in the pH values of the solutions, 
it was assumed that equilibrium was established and the pieces were 
removed and their thicknesses measured again. ‘The results are given 
in Table XVI. The ratio of the final to the initial gauge reading is a 
measure of the degree of plumping of the skin and this is plotted as a 
function of the pH value in Fig. 73. 


TABLE XVI. 
UNHAIRED CALF SKIN IN CONTACT WITH BUFFER SOLUTIONS OF DIFFERENT 
pH VALUuEs. 
Gauge readings in mm. (average of pH value of solution 
duplicates ) at 20° C. 
Initial Final Ratio * Initial Final 
1.421 2.729 1.92 3.96 3.97 
1.205 1.885 1.56 4.14 4.17 
1.269 1.431 pa ike 4.47 4.49 
1.439 1.290 0.90 4.78 et aS 
1.489 1.305 0.88 5.08 5.07 
1.299 1.161 0.89 5.29 . 5-25 
1.347 1.239 0.92 5.57 5-57 
1.388 1.306 0.904 5.78 5.72 
212 1.263 1.04 6.04 6.08 
1.226 Bee ke 1.04 6.20 6.29 
1.391 1.478 1.06 6.48 6.42 
1.248 1.343 1.08 6.69 6.68 
1.435 1.514 1.06 6.96 6.88 
1.292 1.362 1.05 7.08 7.00 
1.379 1.415 1.03 7.41 7.41 
1.413 1.385 0.98 7.68 7.62 
1.393 1.407 1.01 - 7.97 7.89 
1.515 1.520 1.00 8.42 8.44 
1.428 1.427 1.00 8.56 8.50 
1.253 1.343 1.07 9.03 9.13 
1.258 1377 1.09 9.59 9.64 
1.219 1.388 1.14 10.00 9.98 
1.240 1.621 13 10.47 10.51 
1.289 2.206 1.71 11.06 11.08 


* This ratio is a measure of the degree of plumping of the skin, 


The significance of these two points of minimum plumping has 
been discussed in Chapter 5. By comparing Fig. 73, with Fig. 45, 
it will be seen that the plumping of calf skin varies in much the same 
way as the swelling of gelatin with change of pH value. Apparently 
collagen undergoes a change of form, possibly an internal rearrange- 
ment, in passing from an acid to an alkaline solution and the two 
points of minimum represent the isoelectric points of the two forms. 

The degree of plumping at any point between 4.5 and g.O is rela- 
tively so small that the skin would pass as completely bated, if 
judged solely by its fallen condition. Wood, who was probably the 
first to apply the hydrogen electrode to tannery liquors, observed that 
the pH value of fresh dung bate liquors varied from about 4.7 to 
5.4, whereas the bating of a pack of skins raised it to points lying 


BATING 177 


between 6.4 and 8.4. In a lime liquor, which has a pH value of 
about 12.5, the skin is very plump and rubbery. But when it is 
brought into equilibrium with a liquor having a pH value lying between 
4.5 and 9.0, it becomes fallen and flaccid. 

The author has observed that when putrefaction starts in pro- 
tein solutions the pH value of the solution generally tends to shift 
into the region 5.5 to 6.0, regardless of what it may have been in- 


Degree of Plumping of Skin (final/initial gauge reading) 


4 5 6 if 8 c ane) ta Ba ee 
pH Value of Buffer Solution 


Fic. 73.—Showing the two points of minimum plumping of calf skin. 


itially. The putrid dung bates would, therefore, tend to reduce the 
pH value of the limed skin from 12.5 to a value approaching 6. But 
the bate liquor contains phosphates, which act as buffers, and the full 
drop in pH value is prevented. The phosphate is thus a safeguard 
against putrefaction of the skin, which would be quickly damaged if 
the pH value were allowed to drop to the range of maximum rate of 
putrefaction, 


178 THE CHEMISTRY OF LEATHER MANUFACTURE 


Many so-called bating materials probably serve chiefly to reduce 
the pH value of limed skins to the region of minimum plumping. The 
value of this fallen condition is readily apparent for skins which 
are to be tanned in vegetable tan liquors. ‘Tannins diffuse only very 
slowly through swollen skin, but when the skin is in a fallen con- 
dition, the tarinins are enabled to diffuse rapidly into the spaces be- 
tween the fibers, greatly hastening complete penetration. There is a 
fallacy in the assumption that plump leather can be produced only 
by putting skin into the tan liquors in a plump condition. The solidity 
of the resulting leather is determined more by the reaction of the 
liquor itself than by the degree of plumping of the skin when first 
put into the liquor. | 

The manufacture of materials capable of bringing limed skin into 
the condition of minimum plumping is obviously a simple matter. It 
is only necessary to incorporate a buffer material with one which 
will tend to lower the pH value of the limed skin to a final value of 
about 8. Among the materials used for this purpose are boric acid, 
ammonium chloride, weak organic acids and materials yielding acids 
by fermentation, and acid sodium phosphate. The author observed 
five successive lots of skins pass through an artificial bate liquor con- 
taining sodium phosphate, which was entirely uncontrolled, and 0.5 
was the greatest deviation in pH value from the normal value of 8.0 
during the entire period of operation. Whére it is desired only to 
bring the skins into a fallen condition, the process can be carried 
out very effectively using only sodium phosphate and the occasional 
addition of hydrochloric acid to maintain a pH value of about 8. 


Regulation of Hydrogen-Ion Concentration. 


Although the degree of plumping of a skin is a function of the 
hydrogen-ion concentration, the action of a bate liquor in lowering 
the pH value of limed skin has an importance independent of the 
question of plumping. Nearly 80 per cent of the bated weight of a 
skin is due to water, or rather bate liquor. Even though the skin 
may be washed, the water will assume a pH value depending upon 
the substances held in combination with the skin. This adhering 
solution will therefore have an effect upon the tan liquor into which 
the skins are put. If the pH value of this adhering solution is very 
variable, difficulty will be experienced in vegetable tanning because 
the rate of tanning, the rate of diffusion of the tan. liquor into the 
skin, the color value of the tan liquor, and its tendency to oxidize 
are all functions of the pH value. Keeping constant the pH value 
of the solution adhering to the skins entering the tan liquors is a 
factor of great importance and one which made the old dung bates 
almost a necessity to the tanner who had no other way of controlling 
the pH value. The actual pH value, within limits, was probably of 
less importance than keeping it constant at some arbitrary value, which 
could be met by establishing conditions in the tan yard to correspond. 


BATING 179 


Deliming. 


Many persons have looked upon bating chiefly as a process for 
removing the combined lime from the skins. In using a dung bate, 
Wood found from 3 to 6 per cent of lime, calculated as calcium oxide 
on the dry skin, before bating and only from 0.5 to 0.9 per cent after 
bating and all of this appeared to be present as neutral salt. 

Artificial bates, however, do not all have the property of removing 
calcium from the skin. Upon investigating the operation of a bate 
liquor containing phosphates and ammonium chloride and having a 
pH value of 8.4, the author found no diminution of the calcium con- 
tent of the skin during bating, although the skins had become com- 
pletely fallen and practically all of the lime had been converted into 
neutral or insoluble salts. Apparently insoluble calcium phosphate had 
formed in the skin, where it remained. In cases like this, the process 
can hardly be called efficient as a means of deliming. Where nearly 
complete removal of calcium compounds is essential for the best opera- 
tion of later processes, it is much better to employ a properly controlled 
acid liquor, such as those to be described in the next chapter. 


Bacterial Action. 


Bacteria play an important role in the action of dung bates, being 
instrumental in the removal of lime from the skin as well as in lower- 
ing the pH value to the region of minimum plumping. Some of the 
bacteria, or their products, also attack portions of the skin itself, as 
shown by the appearance of nitrogenous matter in solution. In Fig, 74 
is shown a typical plate culture on gelatin? of a dung bate liquor in 
actual use. 

Becker * isolated 54 varieties of bacteria from dog dung and studied 
the actions of many of them upon skin. He found one, which he 
called B. erodiens, capable of producing a falling action of limed skin 
similar to that of the dung bate itself. An artificial bacterial bate 
was developed independently by Wood in England and by G. Popp 
and H. Becker in Germany, but they later joined forces and _ per- 
fected the artificial bate known as erodin, which consists of a nutrient 
material to which a pure culture of B. erodiens is added before using. 
This material has been used on a commercial scale and found to be a 
satisfactory substitute for dung for some kinds of leather. 

Since B. erodiens does not secrete tryptic enzymes, Wood has sug- 
gested adding to it bacteria obtained from the roots of wool in the 
sweating process which secrete a mild form of proteolytic ferment. 
The susceptibility of erodin liquors to become contaminated by for- 
eign bacteria presents an obstacle to any very widespread increase in 
their use. In using erodin, Wood has observed that the fresh liquor 


* Cf. The Properties and Action of Enzymes in Relation to Leather Manufacture. J. T, 


Wood. J. Ind. Eng. Chem. 13 (71621 )Sorr3s. 
* Bacteriological Reactions in the Leather Industry. H. Becker. Z, 6fent, Chem, to 


(1904), 447. 


130 THE CHEMISTRY OF LEATHER MANUFACTURE 


usually has a pH value of about 6.6 and this increases to about 7.3 
during the bating operation. 

Cruess and Wilson ® isolated 10 varieties of bacteria from pigeon 
dung and found that the falling of limed skins could be brought about 
by pure cultures in dilute skim milk. If the bating operation were 
unduly prolonged, the skin proteins became hydrolyzed, but they found 


Fig. 74.—Typical Plate Culture on Gelatin of Puer Liquor. 


that danger from this source could be greatly minimized. by using a 
liquor containing 0.5 per cent of glucose. They pointed out that 
the glucose was decomposed into acids which checked bacterial action 
and assisted in the removal of lime from the skin. 

The prevailing opinion is that bating is not produced directly by 
the bacteria, but rather by the products which they secrete. Of these, 
the enzymes are regarded as the most important because the reduc- 
tion in pH value of the skin, with consequent falling, can be brought 
about by simple chemical means not generally regarded as constituting 
the process of bating. | 


5A Bacterial Study of the Bating Process. W. Cruess and F. H. Wilson. J. Am. 
Leather Chem, Assoc. 8 (1913), 180. : 


BATING 181 


Enzyme Action and Elastin Removal. 


Wood * separated the enzymes from dog dung by precipitation from 
solution with alcohol and showed that the enzymes, in conjunction with 
ammonium compounds, were capable of bating skins. In view of the 
fact that the bate liquor was alkaline, it seemed pretty certain that 
trypsin must be the principal enzyme acting. Wood and Law’ later 
showed that there were at least five different enzymes present in dog 
dung, as follows: 


A peptic enzyme resembling stomach pepsin. 
A tryptic enzyme resembling pancreatic trypsin. 
A rennin (coagulating enzyme). 

An amylolytic enzyme. 

A. lipase. 


baie aa he 


Where a skin contains an abundance of fat cells, the lipase probably 

exerts an important function in hydrolyzing and emulsifying the fats. 
In 1908 Rohm ® patented the use of the enzymes of the pancreatic 
juice and ammonium salts as a bating material. This mixture now 
known as oropon has come into wide use and has largely supplanted the 
dung bates formerly used. 

Recently there has been a concerted effort to determine just what 
part is played by pancreatin in the bating process. As a measure of 
the elastin content of skin, Rosenthal ® used the per cent of nitrogenous 
matter that could be rendered soluble by tryptic digestion. By this 
method he found that bating with oropon reduced the elastin content 
of calf skin from 10.36 to 0.31 per cent, calculated on the dry basis. 
The author’s later investigations of the bating process by means of 
the microscope, however, indicate that Rosenthal’s method of deter- 
mining the elastin content of skin is unreliable. Apparently a large 
portion of the matter included as elastin was derived from the other 
protein constituents of the skin or their hydrolytic products. 

Upon examining a dung bate liquor used to bate sheep grains, 
Wood found that nitrogenous matter had been dissolved equivalent 
to only one per cent of the total protein matter of the skins. As 
nearly as can be judged from microscopic observations, this represents 
approximately the percentage of elastin present in the skin. 

Seymour-Jones *° also suggested that the function of bating is the 
removal of the elastin fibers of the skin. In collaboration with J. T. 
Wood, Seymour-Jones carried out an interesting experiment on the 
bating of sheep skin. The “flywing” grain of a sheep skin was split 
from the main body of the skin, called simply flesh for convenience, 


6 Notes on the Constitution and Mode of Action of the Dung Bate. J. T. Wood. J. 
Soc. Chem. Ind. 17 (1898), tort. 

7 Enzymes Concerned in the Puering or Bating Process. J. T. Wood and D. J. Law. 
J. Soc. Chem. Ind. 31 (1912), 1105. 

®U. S. Pat. 886,411, May 5, 1908. 

® Biochemical Studies of Skin. G. J. Rosenthal. J. Am. Leather Chem. Assoc. 11 (1916), 


3. 
10The Physiology of the Skin. Alfred Seymour-Jones. J. Soc. Leather Trades Chem. 
4 (1920), 60. 


~ 


Fig. 75.—Vertical Section of Calf Skin. 
(After liming and unhairing, before bating.) 


Location: butt. Eyepiece: none. 

Thickness of section: 40 u. Objective: 32-mm. 

Stains: Weigert’s resorcin-fuchsin Wratten filters: B-green; E-orange. 
and picro-red. Magnification: 25 diameters, 


182 


Fig. 76.—Vertical Section of Calf Skin. 
(After bating, before tanning.) 


Location: butt. Eyepiece: none. 

Thickness of section: 40 u. Objective: 32-mm. 

Stains: Weigert’s resorcin-fuchsin Wratten filters: B-green; E-orange. 
and picro-red. Magnification: 25 diameters. 


183 


1834 THE .CHEMISTRY OF LEATHER MANUFACTURE 


and both grain and flesh were cut into halves along the backbone. One 
grain and one flesh were bated with pancreol, a pancreatin preparation 
similar to oropon, while the other halves were delimed with acetic acid, 
but not bated. All four pieces were then tanned with sumac. There 
was comparatively little difference between the bated and unbated 
flesh halves, but the grain samples were very different from each 
other. The bated grain was soft and even, with the hair-holes clean 
and clear, but in the unbated grain the hair-holes appeared to be glued 
up and the surface had a rough, contracted appearance. He con- 
cluded that elastin present in the region of the grain membrane must 
be digested. before tanning in order to produce a satisfactory grain 
surface, but that the bating of the skin under the grain is not only 
unnecessary, but often undesirable. 

The difference which Seymour-Jones found between the two grains 
was probably not due entirely to the bating process, since one was 
treated with acetic acid while the other was not. This means that 
the unbated grain would be subjected to the action of tan liquor at 
a lower pH value than the bated grain. But as the pH value of a 
fresh tan liquor is lowered, there is an increasing tendency for it to 
produce in the grain layer of a skin the rhythmic swelling described 
in Chapter 5. This shows itself first in a roughening of the grain, 
similar to that described by Seymour-Jones, and with further drop in 
pH value the corrugation of the surface appears. The roughening of 
the grain which had not been bated may have been aggravated by the © 
presence of the elastin fibers, but the chief cause was probably the 
lower pH value. 

Wilson and Daub *: ?? undertook to settle definitely the question of 
the removal of elastin in the bating process by means of the microscope. 
They prepared sections of calf skin taken both before and after bating 
with a solution of pancreatin and found that the process removes all 
of the elastin fibers, if sufficiently prolonged. Fig. 75 shows a sec- 
tion of calf skin taken after liming, unhairing, scudding, and wash- 
ing, but before bating. The elastin fibers show as a thick, black band 
just under the grain surface; the magnification here is not sufficiently 
great to show each individual fiber. Another layer of elastin fibers — 
appears at the flesh boundary. The main body of the skin contains 
no elastin fibers excepting those surrounding blood vessels, nerves, and 
muscles. Fig. 76 shows an adjoining section of the same skin taken 
after bating for 24 hours in 0.oI-per cent pancreatin solution at 40° C., 
having a pH value of 7.5. 

The author has recently received a letter from Mr. L. Krall of 
Geneva, Switzerland, claiming priority in discovering, by means of the 
microscope, that the chief function of bating is the removal of elastin 
fibers from the skin. His experiments, performed at the University 
of Geneva from 1914 to 1916, proved that the elastin fibers of skin 
can be entirely removed by digestion in an infusion of dog dung 
at 40°C. His photomicrographs show that the action of dung is 


11 The ‘Mechanism of Bating. J. A. Wilson. J. Ind. Eng. Chem. 12 (1920), 1087. 
2A Critical Study of Bating. J. A. Wilson and Guido Daub, Ibid., 13 (1921), 1137. 


BATING 185 


practically identical with that found by Wilson and Daub for pan- 
creatin, thus furnishing further evidence of the soundness of Wood’s 
conclusion that pancreatin is the active constituent of dung in bating. 
Krall’s important paper ‘* was unfortunately buried in a private bulletin. 

After examining hundreds of sections of skin, taken before and 
after bating, at high magnifications and with the employment of a great 
variety of stains, Wilson and Daub came to the conclusion that the 
removal of elastin is the primary function of bating and that the 
other actions associated with dung bates can all be produced by the 
simple chemical control of the processes other than bating. The fall- 
ing of the skin, however, always accompanies the removal of elastin 
because the range of pH values over which pancreatin acts upon elastin 
is such as to reduce the plumping of limed skin to the point accepted 
as a measure of the completion of the bating process. 

In studying the progress of bating, Wilson and Daub observed cross 
sections of skin taken before and after bating and estimated the per 
cent of elastin removed by the treatment. For this purpose, the sec- 
tions were prepared and stained as described in Chapter 2. The 
enzyme which they employed was a commercial sample of U.S.P. 
pancreatin which showed by analysis: water, 6.3 per cent; ash, 6.8 
per cent; nitrogen, 11.0 per cent; chlorine, 1.7 per cent; phosphates, 


as phosphorous pentoxide, 3.5 per cent; sulfate, none. By the method 


for determining tryptic activity described by Sherman and Neun,’* 
10 milligrams of the sample acting upon I gram of casein in 100 cubic 
centimeters of solution for 1 hour at 40° C. and at pH value of 7.33 
digested 51 milligrams of nitrogen, As a matter of caution, it should 
be pointed out that this does not give a correct measure of the activity 
of the enzyme so far as its power to digest elastin is concerned. The 
author suggests that the elastin-digesting power of a bating material be 
determined solely by the amount of elastin which a given sample can 
digest from skin under rigidly defined conditions. The activity of the 
sample on casein or gelatin may be entirely misleading as regards its 
value as a bating material. 

For each series of experiments, Wilson and Daub cut a piece of 
limed and unhaired calf skin into strips about 2x0.5 inches. There 
is a small, but appreciable, difference in time required for complete 
removal of elastin from skins of different thickness and for this reason 
care was exercised in selecting all strips for any one series from the 
same part of the same skin, so as to have them all as nearly identical 
as possible. Each strip was put into 500 cubic centimeters of liquor, 
a volume large enough to prevent the skin from seriously altering 
the concentration of the liquor. The liquors were all put into dark 
brown bottles to shield them from the light and were kept in a large 
Freas thermostat for the stated lengths of time at 40° + 0.01° C., the 
optimum temperature for most enzyme actions.*° 


18 Ferments in the Tannery. L. Krall. Societe Anonyme, anc. B. Siegfried. Zofingue, 
Switzerland. Private bulletin, June, 1918. 
14H. C. Sherman and D. E. Neun. J. Am. Chem, Soc. 38 (1916), 2199. 
< The Chemistry of Enzyme Actions. K. G. Falk. The Chemical Catalog Co., New 
ork, 


186 THE CHEMISTRY OF LEATHER MANUFACTURE 


Every liquor contained 0.02 mole per liter of added phosphoric 
acid to act as a buffer, in addition to the enzyme, and the potassium 
hydroxide required to give the desired hydrogen-ion concentration. 
The pH value of each liquor was determined both before and after 
the digestion period by means of Hildebrand electrodes and a Leeds 
and Northrup potentiometer, excepting where it was proved by previous 
test that the results obtained by the Clark and Lubs series of in- 
dicators were sufficiently accurate. Except for the more strongly acid 
and alkaline solutions, the change in pH value during digestion was 
practically negligible. Estimates of the per cent of elastin removed 
were made on the basis of removal from the grain layer only. In some 
cases all of the elastin was removed from the grain layer before half of 
it was removed from the flesh layer. Since the shaving operation 
removes practically all of the flesh elastin, its removal in bating is of 
little importance, 

As a rule, a preliminary series covering a very wide range was 
run, followed by a second series covering only the active range of the 
enzyme. A third series was usually run as a check. 


Effect of Hydrogen-Ion Concentration. 


It is well known that the hydrogen-ion concentration is an impor- 
tant factor in determining the rate of digestion by enzymes. Using 
0.1 gram of pancreatin per liter and digesting for 24 hours, complete 
removal of elastin from the skin was obtained only between the pH 
values 7.5 and 8.5. <A portion of the pancreatin was put into a collodion 
sac and dialyzed against running tap water in a dark room for 16 
hours and used in a duplicate series in such quantity as to represent 
0.1 gram per liter of the original pancreatin. The results were identical 
with those obtained with the undialyzed enzyme. A series was then 
run in which the concentration of pancreatin was increased to 1.0 gram 
per liter. Complete removal of elastin was obtained between the pH 
values 5.5 and 8.5. The results of the two series, which are shown 
in Fig. 77, were carefully checked to insure their accuracy. The 
per. cent of the total elastin which was removed js plotted against - 
the pH value of the solution taken after digestion and cooling to 
Bis ; 

The peculiar relation of the curves to each other is significant. 
They nearly coincide at all pH values above 7.5, but at 6.0 the 
stronger solution is still at its optimum activity, while the weaker 
one has apparently entirely lost its elastin-digesting power. When 
an enzyme has been found to have different optimum pH values with 
different substrates, it has been supposed that the effect of the hydro- 
gen-ion concentration upon the substrate has been the determining 
factor. But here we have the same enzyme and the same sub- 
strate, with a change in the optimum range due merely to a change in 
concentration of the enzyme. 


BATING 187 


An explanation is suggested by the work of Northrop,’® ’” who 
has shown that the activity of an enzyme solution is not necessarily a 
function of the apparent total enzyme concentration, but that a portion 
of the enzyme may be inactivated by combining with peptone or other 
foreign matter. He has pointed out further that the extent of the 
formation of addition compounds between protein and enzyme depends 
upon the concentration of protein ion, which in turn is a function 


100 


PANCREATIN PANCREATIN 
PER LITER PER LITER 


Elastin Removed 


Percent. 
iss] 
ro) 


re) 4 5 6 7 8 9 ie) © lab 
pH Value of Solution 


Fic. 77.—Removal of elastin fibers from limed calf skin as a function of hydro- 
gen-ion concentration. Time of digestion 24 hours; Temperature Powe. 


of the hydrogen-ion concentration. If some protein other than elastin 
is responsible for the inactivation of a portion of the enzyme, we should 
expect such action to be a minimum at the isoelectric point of this 
protein. 

After bating, the strips of calf skin were all carefully examined 
for the “bated feel,’ which apparently bears no relation to elastin 
removal, but corresponds to a condition of minimum swelling of the 
skin proteins. The only strips passing this test were those from 
liquors having pH values between 6.1 and 9.8. The average of these 
is 8, which is also the midpoint of the optimum range for the more 

dilute enzyme solution. This value evidently corresponds to the sec- 
16The Effect of the Concentration of Enzyme 6n the Rate of Digestion of Proteins by 


Pepsin. J. H. Northrop. J. General Physiol. 2 (1920), 471. } 
117 The Significance of the Hydrogen-lon Concentration for the Digestion of Proteins by 


Pepsin. “Ibid., 3 (1920), 211. 


188 THE CHEMISTRY OF LEATHER MANUFACTURE 


ond point of minimum plumping of calf skin found by Wilson and 
Gallun and shown in Fig. 73. It is worthy of note that at 40° C. 
Wilson and Daub found no indication of a point of minimum except 
at 8. On the basis of the theory of the existence of two forms of 
collagen, discussed in Chapter 5, it would appear that at 40° Wilson 
and Daub were dealing only with the form stable at higher tempera- 
tures and pH values and whose isoelectric point appears to be at 7.7. 
The following explanation is therefore suggested tentatively. At 
a pH value of 7.7, practically all of the enzyme is left free to attack 


1.0 GRAM PANCREATIN 
PER LITER 


Percent. Elastin Removed 


0.1 GRAM PANCREATIN 
PER LITER 


2 4 6 8 10-12 14 16°15 (20..ceeee 
Hours of Digestion 


Frs. 78.—Removal of elastin fibers from limed calf skiras a function of time 
of digestion. Temperature 40° C.; pH value 7.6. 


the elastin, but as the pH value is decreased and the concentration 
of collagen cation correspondingly increased, more and more enzyme 
is removed from the field of action by combining with it. In the 
weaker enzyme solution at pH = 6 practically all of the enzyme is in 
combination with collagen, whereas in the stronger solution the ex- 
cess of enzyme is still sufficient to digest elastin. It is interesting . 
also to note that Thomas and Seymour-Jones '* found that pancreatin 
attacks collagen at an increasing rate as the pH value is lowered 
irom 8 to 6. In dealing with the effect of hydrogen-ion concentration 
upon enzyme action, it is evidently necessary to know the effect of the 


#8 Hydrolysis of Collagen by Trypsin. A. W. Thomas and F. L. Seymour-Jones. J, Am. 
Chem. Soc. (1923); (advance copy). £ 


oa 7 


apr IE TY, 


BATING 189 


hydrogen-ion concentration upon each substance in contact with the 
solution. 

pik is interesting to compare the optimum pH values for tryptic 
digestion found by other investigators : 19 for albumose Michaelis and 
Davidsohn 20 found 7.7; for casein Sherman and Neun?! found 8.3, 
while Long and Hull *? found 5.5 to 6.3; and for fibrin Long and 
Hull 22 found 7.5 to 8.3. The total range of 5.5 to 8.3 corresponds 
closely to the range found by Wilson and Daub, 5.5 to 8.5, for complete 
removal of elastin by the more concentrated enzyme solution. 


100 


24-HOUR DIGESTION PERIO 


ee ee 
5-HOUR DIGESTION PERIOD 


Percent. Elastin Removed 
on 
ro) 


Hes0.4.10.6 0.8 1,0 2.2 i4 LeG eo G 
Grams Pancreatin per Liter 


Fic. 79.—Removal of elastin fibers from limed calf skin as a function of concen- 
tration of enzyme. ‘Temperature Age nse th va nee 7.0: 


At pH values less than 3.0 there was a marked destruction of 
the collagen fibers, evidently due to acid hydrolysis, and the strips 
were much swollen and rubbery, but no removal of elastin could be 
detected. 


Effect of Time of Digestion. 


Two series of solutions were prepared, one in which the concen- 
tration of enzyme was O.I gram per liter and the other in which it 


was 1.0. All members of both series were otherwise identical. The 


19 Cf, Falk, loc. cit., p. 66. 

20 Biochem. Z. 36 (1911), 280. 

21J, Am. Chem. Soc. 38 (1916), 2203; 40 (1918), 1138, 
22 Tbid., 39 (1917), 1051. 


190 THE CHEMISTRY OF LEATHER MANUFACTURE 


pH value of each liquor was brought to 7.6 and this did not change 
during digestion. Each strip of calf skin was kept in a separate bottle. 
The bottles were removed from the thermostat at fixed intervals during 
24 hours. 

Complete removal of the elastin was effected by the stronger enzyme 
solution in 6 to 8 hours, but in the weaker solution 24 hours were 
required. The progress of the digestion is shown in Fig. 78. The 
time required to start the digestion, 2 hours for the stronger and 5 
hours for the weaker solution, was apparently the time required for 


O.1 GRAM PANCREATIN PER LITER 


Percent. Elastin Removed 
ra 


a 2 3 4 5 6 7 8 9 


Grams Ammonium Chloride per Liter 


Fic. 80.—Removal of elastin fibers from limed calf skin as a function of the 
concentration of ammonium chloride. Time of digestion 24 hours; Temper- 
ature 40° C.; pH value 7.6. 


the enzyme to diffuse into the region of the skin containing the elastin 
fibers.. As will be seen from Fig. 82, these begin about 0.1 millimeter 
‘below the grain surface. 


Effect of Concentration of Enzyme. 


Two identical series of solutions were prepared in which the in- 
dividual members differed only in concentration of pancreatin. One 
series was kept in the thermostat for 5 hours and the other for 24 
hours. The results are shown in F ig. 79 and furnish a study in economy, 
Complete removal of elastin is effected by 0.1 gram of pancreatin in 
24 hours or by 1.1 grams in 5 hours, 


ae ea = ee 
. 


BATING 191 


Effect of Concentration of Ammonium Chloride. 


A study of bating would not be complete if it did not include 
the effect of ammonium chloride, one of the most abundant constitu- 
ents of commercial bating materials. Aside from its use as a filler, 
it has been assumed to be beneficial in removing lime from the skins 
and tending to maintain a slight alkalinity favorable to tryptic diges- 
tion. Two series of solutions were prepared in which the concentration 
of enzyme was 0.1 and 1.0 gram per liter, respectively. To each 
successive member of each series increasing amounts of ammonium 
chloride were added and the pH values of all members were brought 
to 7.6. The time of digestion was 24 hours. The results are shown 
in Fig. 80. 

In working with very dilute enzyme solutions, a distinct activating 
effect was noted upon the addition of 0.5 gram per liter of am- 
monium chloride, while larger amounts showed an inhibitory effect. 
With thin calf skin the activating effect was not detectable with the 
solution containing 0.1 gram per liter of enzyme after a 24-hour diges- 
tion period, because all of the elastin was removed without adding 
any ammonium chloride. In order to show the activating effect in 
these experiments, strips from heavier skins were used, which re- 
quire a somewhat longer time for complete removal of elastin under 


fixed conditions. The activating effect of 0.5 gram of ammonium 
chloride per liter and the inhibitory effect of greater concentrations 


are very marked. It is also important to note that the effect of 
ammonium chloride can be entirely overcome by a sufficient excess of 
enzyme. , 

This behavior of ammonium chloride is interesting in view of the 
finding of Thomas ?* that potassium bromide in concentrations of 0.0 
to 0.1 mole per liter has an inhibitory effect upon the action of malt 
amylase, but in greater concentration has an activating effect. 

At concentrations greater than 50 grams per liter the ammonium 
chloride exerted a destructive action upon the collagen fibers, probably 
due to the formation of free ammonia. 


Distribution of Elastin Fibers in the Skins of Different Animals. 


It is well appreciated by tanners that skins of different animals 
and of animals of different ages must be treated differently in bating, 
as well as in other processes. It has been noted, for example, that 
the bating of a cow hide is less effective than the bating of a calf skin 
under the same conditions. The reason for this will be made ap- 
parent by comparing Figs. 81 and 82, both of which were photo- 
graphed at exactly the same magnification. They represent the upper 
portions of the skins taken after liming, unhairing, scudding, and 
washing, but before bating. Fig. 81 is from a full grown cow hide, 


23 A Noteworthy Effect of Bromides upon the Action of Malt Amylase. A. W. Thomas. 
J. Am. Chem. Soc. 39 (1917), 1501. 


Fig. 81.—Vertical Section of Thermostat Layer of Cow Hide. 
(After liming and unhairing, before bating. ) 


Location: butt. 
Thickness of section: 20 un. 
Stain: Daub’s bismarck brown. 


192 


Eyepiece: 5X. 

Objective: 16-mm. 

Wratten filter: C-blue. 
Magnification: 140 diameters. 


f 


~ 
Ax, 


gn Prs. 
pS 
ge 


Fig. 82.—Vertical Section of Thermostat Layer of Calf Skin. 
(After liming and unhairing, before bating.) 


Location: butt. Eyepiece: 5X. 
Thickness of section: 20 uw. Objective: 16-mm. 
Stain: Daub’s bismarck brown. Wratten filter: C-blue. 


Magnification: 140 diameters. 


; 193 


Mtoe ane 
tA 


Fig. 83.—Vertical Section of Thermostat Layer of Sheep Skin, 
(After liming and unhairing, before bating.) 


Location: butt. Eyepiece: 5X. 
Thickness of section: 20 u. Objective: 16-mm. 
Stain: Daub’s bismarck brown. Wratten filter: C-blue. 


Magnification: 140 diameters, 


194 


in. 


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oA wn 
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Hm 8 
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S =P 
“a EGS 
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BV ofa 
no O.2 Fee 
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3 aoexes 
fe See a 
ois Oe & 
5 One 
a 
airs 
pot 
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as 
Sas 
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95 


I 


196 THE CHEMISTRY OF LEATHER MANUFACTURE 


while Fig. 82 is from a young heifer calf skin. It will be noted 
that the older skin has relatively fewer elastin fibers, although they 
extend into the skin to a greater absolute depth. This greater depth 
necessitates leaving the hide in the bate liquor for a longer time, so 
that the enzyme may diffuse to the most deeply seated fibers, but, on 
the other hand, there is less reason for removing the elastin fibers from 
the heavier skin, because they are relatively fewer. : 
Fig. 83 shows the elastin fibers of a sheep skin before bating and 
Fig. 84 those of a hog skin. The elastin fibers of the hog skin are 
very sparsely. scattered; the heavy band of elastin fibers passing 
obliquely upward to the right, across the center, is apparently there 
for the purpose of protecting the erector pili muscle, which it surrounds. 
Figs. 81, 82, 83, and 84 should be compared with Figs. 11, 18, 
28, and 30, respectively, of Chapter 2, which show sections taken from 
the same skins when fresh. 


Effect of Elastin Removal on the Final Leather. 


Wilson and Daub attempted to determine the practical value of 
bating by comparing bated and unbated skins. A limed calf skin was 
cut into halves along the line of the backbone, the elastin was com- 
pletely removed from one half by means of pancreatin, while the 
other half was simply treated with dilute ammonium chloride solu- 
tion having a pH value of 8, in order to reduce its degree of plumping 
to that corresponding to what is accepted as the bated state. Both 
halves were then thoroughly washed. It was recognized that an exact 
comparison of the two halves during tanning could not be made, if 
the pH values of the absorbed solutions were very different. Every 
effort was made to have the pieces identical, excepting for elastin 
content. 

The most noticeable difference was observed during the early stage 
of vegetable tanning. The surface layers of the skin naturally tan 
‘ more rapidly than the fibers in the interior and there is a tendency 
for the grain surface to expand temporarily to a greater extent than 
the rest of the skin. The elastin fibers in the unbated half evidently 
tended to prevent this expansion and the result of the tension pro- 
duced was a slightly harsh feel, although the grain appeared tight 
and smooth to the eye. The grain of the completely bated half, 
however, actually expanded, giving the skin temporarily a wrinkled 
appearance, although the grain felt very soft and silky. When both 
halves had become completely tanned, this difference had almost dis- 
appeared. In the finished leather, the only difference in appearance 
was a slightly lighter color in the bated half. Photomicrographs of 
exactly corresponding points on the grain surface of the two halves 
are shown in Fig. 85. The difference in appearance of the grain sur- 
face in the two cases is practically negligible. In carrying out prac- 
tical tests of this kind, tanners usually fail to appreciate the im- 
portance of having the test pieces in equilibrium with solutions of 


BATING 197 


the same pH value and often attribute to differences in bating dif- 
ferences in the properties of the leather actually caused by differences 
in pH value. | 
While bated and unbated finished leathers appear much alike to 
the eye, there are perceptible physical differences, such as one might 
expect to find in view of the fact that the elastin fibers have been 
removed from under the grain of the bated leather. The desirability of 
completely, or even partially, removing elastin from skin depends 
upon the use to which the leather is to be put. Bated leathers are 


Not Bated 


Fig. 85.—Grain Surfaces of Tanned Calf Skin. 


Eyepiece: none. Wratten filter: K2-yellow. 
Objective: 48-mm. Magnification: 7 diameters. 


usually a little softer than unbated leathers, but this is desirable for 
some leathers and undesirable for others. Wood ** believes that. it 
is not necessary, or even desirable, to remove all of the elastin in bating, 
but that it is sufficient for the elastin fibers to be broken up or 
weakened, in order that the desired suppleness may be obtained. 


Digestion of Collagen during Bating. 


Although Thomas and Seymour-Jones have shown that pancreatin 
hydrolyzes collagen, the work of Wilson and Daub indicates that no 
serious loss of collagen occurs where the pH value is kept within 
the limits 7.5 to 8.0 and the action is stopped just as soon as all 
of the elastin fibers have been dissolved. Unduly prolonging the 
bating operation is sure to result in a very considerable hydrolysis 
of collagen, with corresponding decreases in the yield and firmness 
of the leather. Often an apparently heavy loss of collagen during 


24 The Properties and Action of Enzymes in Relation to Leather Manufacture. Loc. cit. 


198 THE CHEMISTRY OF LEATHER MANUFACTURE 


bating may be attributed to a previous breaking down of the collagen 
by excessive liming, putrefaction, or contact with liquors containing 
much ammonia. Manufacturers of glove leather sometimes make use 
of these agencies in order to get a very soft leather. They leave the 
skins in the lime liquor until a considerable amount of hydrolysis 
of collagen has taken place and then subject the skins to a prolonged 
bating. Much valuable collagen is thus lost, but the skins are thereby 
rendered more suitable for a specific purpose. 


Chapter 10. 
Drenching and Pickling. 


In the final preparation of the skin for tanning, the pH value 
of the solution absorbed by the skin and with which the skin is in 
equilibrium must be adjusted to suit the particular method of tanning 
to be employed. During liming, this solution has a pH value of about 
12.5; during bating, a pH value of about 7.5. Before skins can be 
tanned properly by any of the common methods of tanning, the pH 
value of this solution must be lowered considerably below the value 
7-5. During vegetable tanning, the pH value of the liquor is usually 
less than 5 and in chrome tanning less than 4. By using tan liquors 
containing the proper excess of acid, the adjustment of pH value may 
be made in the tan liquor itself. But this is often a very difficult 
matter where the process is not under rigid chemical control. 


Drenching. 


For certain classes of leather, it is customary to subject the bated 
skins, before tanning, to a process known as drenching. Sometimes 
the bating process is omitted, as entirely unnecessary, and the skins 
are drenched directly after the washing following the unhairing process. 
The drench liquor is prepared by mixing 5 to 10 grams of bran per 
liter of water at 30° to 35° C. and allowing the mixture to ferment, 
with the formation of organic acids. The skins are put into this 
liquor contained in a vat equipped with a paddle wheel which keeps 
the liquor well stirred. In some tanneries, the fermentation is carried 
out in special tanks and only the clear, decanted, acid solution used 
on the skins. The acid: dissolves any lime remaining in the skin and 
brings the skin into a more suitable condition for tanning. The particles 
of bran also exert a sort of cleansing action upon the skin, tending 
to absorb dirt and greases. The treatment is usually continued for 
several hours, but the completion of the process is determined by 
skilled workmen, who have learned to judge by the feel and appear- 
ance of the skin just when it is ready for the particular tanning process 
to be employed. | 

During the process, there is a considerable evolution of gas, which 
tends to cause the skins to float to the surface. In a drench in actual 
use, Wood * found that the gases had the following composition: 

7 The Properties and Action of Enzymes in Relation to Leather Manufacture. J, T. 
Wood. J. Ind. Eng. Chem. 13 (1921); 1135. 

199 


200 THE CHEMISTRY OF LEATHER MANUFACTURE 


Carbon dioxide 25.2 per cent 
Hydrogen sulfide trace 
Oxygen 2.5 
Hydrogen 40.7 
Nitrogen 26.0 


The acids produced per liter were 


Formic 0.0306 gram 
Acetic 0.2042 
Butyric 0.0134 
Lactic 0.7907 


Only an insignificant quantity of other materials were formed during 
drenching, trimethylamine being the chief. 

It was found that the starch of the bran is converted into glucoses 
and dextrin by the action of an amylolytic enzyme, cerealin, discovered 
by Mege Mouries.? It resembles the diastase of translocation de- 
scribed by Brown and Morris? in their work on the germination of 
grass seeds. It transforms starch into dextrin and glucose, whereas 
ordinary malt diastase transforms starch into dextrin and maltose. 
The action of cerealin is much slower than that of diastase. The 
sugars are then fermented by bacteria (Bacillus furfuris) with the 
formation of the organic acids listed above. The principal acid pro- 
duced is lactic; the acetic acid is produced directly from the glucoses 
without any preliminary alcoholic fermentation by yeasts. 

In the hands of experienced operators, the drenching process sel- 
dom gives much trouble, but it is not quite foolproof. If the acidity 
of the liquor increases rapidly and the skins are not removed in time, 
they become excessively swollen and may even be destroyed by hydro- 
lysis, especially if the liquor is very warm. How much enzymes play 
a part in this hydrolysis is not yet known. Apparently danger from 
this source can be prevented by adding salt to the liquor to repress 
‘the swelling of the skin just as soon as it becomes very noticeable. 

In his review of the damage to skins that may be caused by im- 
proper control of the drenching operation, Wood * points out that the 
discovery of the effectiveness of salt in preventing the destruction 
of skin in an acid liquor that would otherwise cause excessive swelling 
represents the origin of the modern pickling process. 

Sometimes the fermentation may not proceed in the usual manner 
and the liquor, instead of becoming acid, turns slightly alkaline, fre- 
quently becoming bluish black, due to the presence of chromogenic 
bacteria. Under these conditions the skin is rapidly attacked by 
proteolytic organisms, but may be saved if transferred in time to a 
solution of acid and salt. 

When the fermentation is accompanied by a very rapid evolution 

* Compt. rend. 37 (1853), 351; 38 (1854), 505; 43 (1856), 1122; 48 (1859), 431; 50 
es 4067, 


Chem, Soc. 57 (1890), 458 
4 Bacon Bating and Drenching of Skins, p. 237. 


DRENCHING AND PICKLING 201 


of gas, the skins may be damaged by the formation of gases inside 
of the skin which burst out through the grain surface, leaving small 
holes. A damage very similar in appearance may be caused by pro- 
teolytic bacteria developing on the grain surface, each colony forming 
a small hole. This usually results from operating the drench at too 
high a temperature. A high temperature, especially in the presence of 
an excess of acid over that normally present, may result in a con- 
siderable amount of hydrolysis of collagen and the leather will feel 
rather spongy and empty. 

When bacteria attack the grain during drenching, the surface of 
the finished leather may show dull patches, as though it were etched. 
In one instance, Eitner® found that this was caused by Bacillus 
megaterium, which formed a slimy film over the grain surface, which 
was attacked by a proteolytic enzyme secreted by the bacillus. 

Wood and Wilcox ® showed that if the acids ordinarily found in 
the drench are used in pure solution in the proportions in which they 
occur in the drench, the action upon the skin is the same, except for 
being more rapid. With the appreciation of the fact that the active 
constituent of the drench is the acid formed, tanners began to sub- 
stitute pure solutions of organic acids, such as lactic and acetic. These 
could be used with safety, simply by adding the acid at such rate as 
to keep the solution just neutral to methyl orange. Hydrochloric acid, 
being cheaper, is often used, although it makes the control more deli- 
cate. In this way practically all of the lime can be removed from 
the skins and the skins then combine with a sufficient amount of the 
acid so that they do not reduce the acidity of the ordinary vegetable 
tan liquor into which they may be put. 

But even when pure solutions of acid were employed to drench 
skins, no fixed rule could be made for all tanneries. If the vegetable 
tan liquors contained a considerable amount of salt and other soluble 
nontannins, the drench could be operated at a lower pH value with 
safety. Where fresh liquors of tanning materials containing a relatively 
small proportion of nontannin were used, there was danger of the 
skins being damaged by the rhythmic swelling described in Chapter 5, 
whenever the pH value of the drench fell below some fixed value, 
which depended upon the composition of the tan liquor employed. 
This trouble can be avoided by the addition of salt to the tan liquor, 
but the remedy may be almost as undesirable as the disease, since 
many tan liquors are precipitated by the addition of salt. In gen- 
eral, the purer the first tan liquor into which the skins are put after 
drenching, the more delicate must the control of the drenching 
operation be. 

It sometimes happens that the tan liquors employed contain easily 
fermentable sugars, which are continually being converted into organic 
acids. In such cases, the use of a drench prior to tanning may be 
undesirable and even the bating operation may be unnecessary, where 


5 Gerber (1898), 204. 
6 Further Contribution on the Nature of Bran Fermentation. J. T. Wood and W. H. 
Wilcox. J. Soc. Chem, Ind, 12 (1893), 422. 


202 THE CHEMISTRY OF LEATHER MANUFACTURE 

the removal of elastin is not important. The tan liquor itself actually 
becomes a drench and the lime salts formed serve to prevent rhythmic 
swelling. Where the skins have been drenched prior to putting into 
the tan liquor, the acid present may prove excessive and the skins will 
be spoiled. 

One tanner may employ a non-acid tan liquor preceded by a drench, 
another may use acid tan liquors and do away with the drenching 
operation, and yet both may produce the same kind of finished leather. 
But one would not dare to adopt only a part of the other’s methods, 
which might prove disastrous; he must adopt all or none. This will 
serve to explain why it is not possible to outline quantitatively a rigid 
system of bating, drenching, deliming, or any other process, so that it 
may be used in any tannery. All fundamental operations in any one 
tannery are interdependent and a change, even one for the better, in 
one operation might necessitate a corresponding change in nearly every 
other operation. | 


Pickling. 


The pickling operation differs from drenching chiefly in the fact that 
salt is used in conjunction with the acid. Formerly it was the cus- 
tomary practice to soak the limed or bated skins in a vat containing 
dilute sulfuric acid until they became somewhat swollen and then 
to transfer them to a saturated solution of sodium chloride, which 
repressed the swelling. Now it is more common to use the acid and 
salt in solution together, the preliminary swelling having been found 
unnecessary and sometimes undesirable. A satisfactory pickle liquor 
for most purposes consists merely of a molar solution of sodium 
chloride to which sulfuric acid is added in the desired amounts. 


Pickle liquors are used for a number of different purposes, the . 


chief of which are the preparation of skin for chrome tanning and 
the preservation of unhaired skins so that they may be kept for an 
indefinite period before tanning. 

In preparing skins for chrome tanning, the concentration of acid 
most desirable to use depends upon the degree of basicity of the chrome 
liquor employed. The more concentrated the acid in the pickle liquor, 
the more quickly does the system tend to reach a condition approxi- 
mating equilibrium. Furthermore, the more concentrated the acid 
solution absorbed by the skin, the more quickly will the chromium 
salts penetrate into the interior of the skin during the tannage. On 
the other hand, if the concentration of acid is too great, the rate of 
fixation of chromium by the skin will be reduced to an undesirable 
degree, unless the excess of acid is neutralized by the addition of sodium 
bicarbonate, borax, or other agent, during the tannage. 

Pickling has the advantage over drenching that it is extremely easy 
to control chemically. If the concentration of salt is not allowed 
to fall below half-molar, the pickle liquor can be controlled by simple 
titrations, using methyl orange as indicator. Regardless of the variable 
amounts of lime which the skins may contain before pickling, they 


DRENCHING AND PICKLING 203 


can all be brought into a uniform condition simply by so regulating 
the concentration of acid that all skins finally reach equilibrium with 
solutions of the same concentration. When used in this way, the 
pickling process becomes a stabilizer of inestimable value in chrome 
tanning. 

When the equilibriumi concentration of acid is maintained at 0.05 
normal or greater, the pickling of light skins requires only a few 
hours, but for weaker solutions and for heavy hides, the stock must 
remain in the liquor over night. In acid solutions greater than 0.01 
normal, there is practically no danger of the skins being attacked by 
bacteria. The salt present is sufficient to prevent undue swelling at 
any pH value so that the process may be considered entirely safe, if 
only ordinary care is used. 

For preserving skins, after bating, it is sufficient to bring them 
into equilibrium with a solution containing I mole of sodium chloride 
and 0.01 mole of sulfuric acid per liter. The liquors may be used for 
several consecutive lots of skin as the calcium sulfate formed is soluble 
in acid solution. The skins are usually pickled in vats equipped with 
paddle wheels, which keep the skins and. liquor in motion, greatly 
hastening the attainment of equilibrium. After equilibrium has been 
established, the skins are withdrawn from the liquor and thrown 
over wooden horses to drain. They may then be kept in a damp 
condition for many months. 

It is often desired to tan such skins later in vegetable tan liquors 
of such composition that they would be precipitated by the salt and 
acid present in the skins. In such cases, the skins are first depickled 
by soaking in paddle vats containing a solution of half-molar sodium 
chloride to which borax is added at such rate as to keep the solution 
neutral to methyl orange. When equilibrium has been established, 
the skins are transferred to a wash wheel and the salt washed out by 
means of running water. They are then ready for tanning. Depickling 
is unnecessary in the case of chrome tanning. 

In the control of pickle liquors, it must not be assumed that the 
_ decrease in concentration of acid is caused only by its neutralization 
' by lime. Two other factors contribute to the decrease. The bated 
skins usually contain about 80 per cent by weight of water, only 20 
per cent representing collagen. Part of the decrease is caused by 
the dilution by this water. The author has found that 1 gram of col- 
lagen combines with approximately 0.00133 gram equivalent of acid. 
By making allowance for the decrease in concentration of acid caused 
by dilution and by combination with the collagen, the amount consumed 
in neutralizing lime can be roughly approximated. 


Chapter II. 
Vegetable Tanning Materials. 


It has been known since prehistoric times that raw skin is colored 
and rendered imputrescible by contact with aqueous solutions of ma- 
terials obtained from many forms of plant life. The active principle, 
- which is widely distributed throughout the vegetable kingdom, is a 
class of complex organic compounds known as tannin. By vegetable 
tanning is meant the combination of tannin with the protein matter of 
skin to form leather. 

Among the materials which have assumed commercial importance 
as a source of tannin for leather manufacture are barks, woods, leaves, 
twigs, fruits, pods, and roots. Tanning extracts obtained from differ- 
ent sources show very different properties, which is due in a large 
measure to the foreign matter extracted with the tannin. 


Classification. 


Many attempts have been made to classify tanning materials ac- 
cording to their behavior in tanning practice, but this varies so widely 
with the nature and proportions of foreign matters extracted with the 
tannin that attempts at classification on this basis have not yet re- 
sulted in any scheme of great practical value. The properties of a 
tanning extract depend more, in many cases, upon the method of 
extraction or the conditions under which it is used in the tannery 
than upon its source in nature. By suitably controlling the conditions 
of tanning, it has been found possible to get practically the same result 
from tanning materials otherwise exhibiting markedly different 
properties. | 

The tannins themselves, however, seem to fall chemically into two 
general classes, which have been named pyrogallol and catechol from 
the fact that tanning materials usually yield the one or the other of 
these two substances upon dry distillation. Upon fusion with sodium 
hydroxide, the pyrogallol tannins yield sodium gallate while the catechol 
tannins yield sodium protocatechuate. The pyrogallol tannins con- 
tain about 52 per cent of carbon as against about 60 per cent in the 
case of the catechol tannins. The two classes exhibit a number of 
different properties by which they may be differentiated. 

All tannins seem to possess in common the property of precipitat- 
ing gelatin from solution and this is used as a test to indicate the - 
presence of tannin in solution. ‘The reagent is made by dissolving 10 

204 


VEGETABLE TANNING MATERIALS 205 


x 


grams of gelatin and 100 grams of sodium chloride in 1 liter of water. 
One drop of the gelatin-salt reagent is added to 5 cubic centimeters of 
the solution suspected of containing tannin. Under ordinary conditions, 
a precipitate is formed if more than a trace of tannin is present. The 
sensitivity of this test and the conditions under which it may fail to 
operate will be discussed in Chapter 12. 

When ferric salts are added to tannin solutions, a deep blue color 
is formed in the presence of pyrogallol tannins and a deep green in 
the presence of catechol tannins. All tannins are precipitated by lead 
acetate, but if the solution is first made approximately normal to acetic 
acid, the pyrogallol tannins only are precipitated by the addition of 
lead acetate, the catechol tannins remaining in solution. On the other 
hand, the catechol tannins are precipitated by the addition of an excess 
of bromine water, while the pyrogallol tannins remain in solution. 

A common method for differentiating between pyrogallol tannins 
and those of the catechol group is to add 10 cubic centimeters of 40-per 
cent formaldehyde solution and 5 cubic centimeters of concentrated 
hydrochloric acid to 50 cubic centimeters of the tannin solution and 
to boil the mixture for half an hour in a flask fitted with a reflux 
condenser. Catechol tannins are completely precipitated by this treat- 
ment. The solution is cooled and filtered. To 10 cubic centimeters 
of the filtrate are added 5 grams of sodium acetate crystals and 1 cubic 
centimeter of a I-per cent iron alum solution. A strong bluish violet 
coloration will appear if pyrogallol tannins are present, but none if 
the original solution contained only catechol tannins. 

The separation of the tannins into these two groups and the exten- 
sive studies made of the reactions of the many different kinds of 
tanning materials have furnished the basis for a scheme of qualitative 
recognition of vegetable tanning materials which is sometimes of value 
in detecting adulteration in commercial tanning extracts. One of the 
best of these qualitative schemes is that of Procter.* 

When liquors containing pyrogallol tannins undergo fermentation 
in the tan yard, they usually deposit finely divided ellagic acid, which 
appears as sludge in the bottom of the vat or as bloom on the surface 
of the leather. Catechol tannins, on the other hand, yield a difficultly 
soluble material called reds, or phlobaphenes. 


Sources of Tanning Materials. 


Only the more important raw materials will be mentioned here; for 
more comprehensive lists, the reader is referred to the standard work 
of Dekker 2 and to the books of Procter* and Harvey.* Among the 
barks used most widely as a source of tannin are those of the several 
varieties of oak. Oak bark is one of the few materials furnishing both 
pyrogallol and catechol tannins, although the latter predominate. Tan- 

1 Leather Chemists’ Pocket-Book. H. R. Procter. E. & F. N. Spon, London (1912). 

2 Tanning Materials. J. Dekker. Verlang von Gebrtider Borntraeger, Berlin (1913). 


8 Principles of Leather Manufacture. H. R. Procter. D. Van Nostrand Co., New York 


(1922). 
Tanning Materials. A. Harvey. Crosby Lockwood & Son, Londen (1921). 


206 THE CHEMISTRY OF LEATHER MANUFACTURE 


ning extracts obtained from oak bark have long been favorites for the 
production of leather where firmness and solidity are desired. Hem- 
lock bark is used extensively in the United States for the manufacture 
of heavy leathers. Extracts made from the barks of the larch, spruce, 
and fir are used to a very considerable extent both in America and in 
Europe. The barks of the mimosa, or wattle, the mallet, and the several 
species of mangrove, which are grown in Australia and South Africa, 
are very rich in tannin. Babool bark is commonly used in India and 
willow and birch in Russia. The leather known as Russia calf was 
originally tanned with birch bark, to which it owed its characteristic 
odor. As a general rule, the tannins of the barks belong to the 
catechol group. 

Among the woods, that of the quebracho, grown in South America, 
is probably richest in tannin, ‘The tannins of chestnut and oak woods 
find application in the manufacture of sole leather for blending with 
other materials. Quebracho tannin is of the catechol type, while that 
of chestnut and oak woods is of the pyrogallol type. The extract 
obtained from the cutch wood of India is widely used as a mordant jn 
the dyeing of leather, Recently an extract of the wood of the osage 
Orange tree has appeared on the market both as a natural dyestuff 
and as a tanning agent. 

The most important extracts obtained from leaves and twigs are 
those of the gambier of India and the sumac of Sicily. The stoner 
belongs to the catechol and the latter to the pyrogallol group. Gambier 
is one of the mildest tanning materials known, a property which it 
apparently owes to the large amount of nontannins present in the 
extract. It is used as a mordant and, in mixtures with other materials, 
in the manufacture of light leathers. Sumac is commonly used to 
tan the grain splits of sheep skins for hat bands, etc., and as a mordant. 
It is rather easily decomposed by boiling water. 

A variety of unripe nuts and pods form a much used source of 
tannin, usually of the pyrogallol type. These often contain easily 
fermentable sugars and, by their use in tanning, it is often possible to 
do away with the acid drench to which skins are sometimes subjected 
prior to tanning. The light colors obtained when using materials like 
these, which yield acids by fermentation, may be explained, in the 
light of recent Investigations, by the fact that the color of a tan liquor 
as well as that of the leather it produces becomes lighter the lower the 
PH value. The pods of the algarobilla and divi-divi, grown exten- 
sively in Central and South America, and the dried, unripe nuts of the 
myrobalan tree of India are used jn mixtures with other materials that 
do not yield acids so readily. In the preparation of some mixtures, 
valonia, from the acorn cups of the Turkish oak, is favored. Another 
easily fermentable tanning extract is obtained from the babool pods 
of India, which contain both pyrogallol and catechol tannins. 

Among the roots used as a source of tanning extracts, those of the 
palmetto, grown in the United States, and the canaigre, grown in 
Mexico and Australia, are perhaps the most common. The latter is 
rich in catechol tannin and has a tendency to ferment rather easily. 


VEGETABLE TANNING MATERIALS 207 


Where there is no chemical control of the tan liquors, the selection 
of tanning materials must be governed by the nature of the operations 
preceding and following the tanning process, as well as by price and 
availability of the materials. While quebracho extract, for example, is 
an excellent tanning material, its pure solutions are hardly suitable for 
receiving consecutive lots of raw skin containing much lime. Their 
naturally low acidity would be quickly neutralized and the tannin would 
then be precipitated by the lime, or oxidized, and cease to tan properly. 
But this danger would be greatly lessened by the use of a mixture of 
tanning extracts containing acid-producing materials, like those in divi- 
divi or myrobalans. 


Leaching. 


It is still common to find tanneries equipped to extract the tannin 
from the raw materials grown in neighboring districts, although the 
manufacture of tanning extracts has now become a separate industry, 
which has proved useful in making a greater variety of materials avail- 
able to the individual tannery. 

One of the oldest systems for leaching raw materials, and the one 
most commonly used in tanneries, is known as the open vat method. 
The bark, or other material, is broken into small pieces and then 
shredded in a bark mill. The leaching tanks are usually arranged in 
batteries of about eight and are fitted with perforated false bottoms on 
which the bark is placed. The bottom of each tank is fitted with a pipe 
through which liquor may be drawn off or pumped from one tank to 
another. When fresh bark is put into a given tank, liquor is run onto 
it which has been used to leach the bark in all of the seven other tanks. 
This strong liquor is finally drawn off and pumped into a storage tank. 
The bark is then leached with liquor which has passed through only 
six other tanks. The eighth leaching of this bark is made with fresh 
water, after which the bark is dumped and discarded. 

Fresh water is used to leach only the most nearly exhausted bark. 
As the liquor becomes stronger in tannin, it is run onto fresher bark, 
and finally onto the previously unleached bark. As soon as each tank 
is dumped, it is again filled with fresh bark and becomes the head 
vat in the cycle, which is continuous. The object of this system of 
leaching is to get final liquors as concentrated as possible. In the 
tannery, the liquor in the storage tank is used as needed, but in the 
extract plant it is necessary to evaporate off most of the water so as 
to make its subsequent transportation practical. 

The extraction of the raw material is often facilitated by the use 
of mechanical devices. Sometimes the leaching tanks are equipped 
with mechanical stirrers or with pipes for bubbling air up through 
the liquor. In another system, the tanks are replaced by revolving 
drums, used on the same principle as the open vats, the liquor being 
pumped from one drum to another. In still another system, the bark, 
or other material, is forced through a trough in one direction, by 
means of a screw conveyor, while water flows over the bark in the 


208 THE CHEMISTRY OF LEATHER MANUPACl Ure 


opposite direction. At the point of entry of the fresh water, the bark 
is practically exhausted and is dumped onto a pile from which it is 
subsequently moved to the furnaces for fuel, or is disposed of in some 
other way. At the point of entry of the bark, the liquor is richest in 
tannin and is conducted to the storage tank. 

In many extract plants, autoclaves are employed in order to leach 
the bark under pressure, which increases the yield obtained. The liquor 
is pumped from one autoclave to another, just as in the open vat 
system. In a system only recently devised, the raw materials are 
leached in autoclaves under a vacuum. ‘The advantage claimed for this 
is that the liquor may be kept boiling at a very low temperature, 
giving increased yields, but not at the expense of the quality of the ex- 
tract. The relative merits of the pressure and vacuum systems will 
probably be brought out more clearly when they have been more 
thoroughly investigated. 


Effect of Temperature. 


The rate at which tannin can be extracted from the raw material 
increases with the temperature of the water used, but so also does the 
rate at which the dissolved matter decomposes. The variation of the 
ratio of these two rates with temperature determines the optimum 
temperature that it is desirable to employ and this is different for 
different materials. It is customary to extract the fresh material at 
a low temperature and to increase the temperature of extraction until 
the material is practically exhausted. In using the open vat system 
for ordinary barks, it is a good plan to have the fresh water at the 
boiling point and to allow its temperature to fall slowly to about 60° C. 
as it passes over fresher bark. The temperature of the liquors can be 
controlled by having suitable heating coils placed in the tanks just 
under the false bottoms. : 


Effect of Hardness and Alkalinity of the Water. 


When a very hard, alkaline water is used in leaching, the tannin 
yield is very low and the extract is dark in color and of poor quality. 
This has been the subject of numerous investigations, from which the 
general conclusion has been drawn that the use of a soft water in 
leaching is imperative. But the recent work of Wilson and Kern seems 
to indicate that the question of hardness of the water used is of 
less importance than the pH value of water and liquor. 


Effect of pH Value on the Color of Tan Liquors. 


Wilson and Kern ° made a special study of the effect of pH value on 
the color of gambier and quebracho liquors. Two tan liquors were 
prepared, one from gambier and the other from quebracho extract. To 


5 The Color Value of a Tan Liquor as a Function of the Hydrogen-Ion C i 
J. A. Wilson and E. J, Kern. J. Ind, Eng, Chem. 13 (1921), 1025. : oT eee 


VEGETABLE TANNING MATERIALS 209 


each was added sufficient phosphoric acid to bring the pH value to 2. 5 
as determined by the hydrogen electrode. The phosphoric acid was 
added to act as a buffer in preventing large changes in pH value 
upon long standing. To equal portions of each, sodium hydroxide 
was added to give series of tan liquors ranging in pH value from 
3.0 to 12.0 and all having a tannin content of 1 per cent, as deter- 
mined by the Wilson-Kern method, to be described in the next chapter. 
The gambier series varied in color from light straw at 3.0 to a very 
deep red at 12.0. The quebracho series was similar in color excepting 
that the liquors of lower pH value had a touch of violet. Either 
series suggested a standard series of colors such as is used in the 
indicator method of determining hydrogen-ion concentration, except 
for the fact that a light precipitate formed in all liquors having a pH 
value of 4.0 or less. The difference in color was evidently a true 
indicator effect, for any member of one series could be made to match 
any other member simply by bringing it to the same pH value. All 
members of either series appeared practically identical when brought 
to a pH value of 3.0. This complete reversibility of color change, 
however, was not found when liquors at higher pH values were allowed 
to stand long exposed to air. 


Effect of pH Value on the Oxi- 
_ dation of Tan Liquors. 


Two complete series of each ex- 
tract were poured into test tubes; 
the tubes of one series of each 
were tightly stoppered, while the 
others were left open to the air. 
Next day the liquors in the stop- 
pered tubes showed practically no 
change, but the others had become 
darker in color, the more so the 
higher the pH value. When the 
liquors in a series not exposed to 
air were all brought to a pH value 
of 3.0, they all assumed practically 
the same color. But when those of 
a series that had been exposed to 


GAMBIER 


QUEBRACHO 


Cubic Centimeters of Precipitate from 100 cc, 
Tan Liquor After Bringing pH Value to 3.0 


air were all brought to 3.0, they 4.5.69 "By S09 33 
did not assume the same color, but PH ence tien 


were darker the higher the pH 
: ; _ Frc. 86—Showing how tendency of a 

value during the period of reoe tan liquor to form a precipitate when 
sacle to air; furthermore a pre- brought to a pH value of 3 varies 
cipitate settled out from those with its pH value during a period of 
whose pH values had been in the exposure to air. 
vicinity of 9. ; 

This precipitate formation is very curious. A complete series of 
each extract was allowed to stand exposed to air in shallow dishes for 
3 days; the liquors were then made up to original volume and poured 


210 THE CHEMISTRY OF LEATHER MANUFACTURE 


into 100-cubic centimeter graduate cylinders. Each was brought to a 
pH value of 3.0 by the addition of hydrochloric acid and allowed to 
stand over night. Next day the volume of precipitate from 100 cubic 
centimeters of original liquor was read from each cylinder. The results 
are shown in Fig. &6. 

Keeping a solution of either extract exposed to air while its pH 
value is 9 causes it to yield an enormous precipitate when its pH 
value is subsequently brought to 3.0. But keeping it exposed to air 
when its pH value is greater than 10 apparently prevents its precipita- 
tion when brought to 3; all such liquors remained brilliantly clear. 
The addition of a great excess of acid, however, caused all liquors to 
precipitate, while any precipitate could be completely redissolved by th 
addition of sufficient alkali. | 

Another interesting fact is that the liquors exposed to air when 
their pH values lay between 8 and 9 gave much trouble with the 
hydrogen electrode. After bubbling hydrogen through them for only 
a few minutes, the voltage would fall rapidly towards zero. Even 
when brought to a pH value of 3.0, the liquors still gave this trouble, 
making it necessary to check the results by means of indicators. No 
such trouble was encountered with liquors exposed to air at pH values 
below 7 or above 10. Apparently pH —9 is a critical point in the 
oxidation of tan liquors. 

The curves in Fig. 86 show that this effect of oxidation is ap- 
preciable at all pH values from 6 to about 10. Most hard waters have 
pH values lying within this range and many of them have pH values 
higher than 8. : 


Effect of pH Value on the Precipitation of Tan Liquors. 


Wilson and Kern ® also studied the effect of pH value on the pre- 
cipitation of quebracho liquors. Four series of solutions of solid 
quebracho extract were prepared according to the official method of 
the American Leather Chemists Association,’ except for the additions 
of sulfuric acid, hydrochloric acid, sodium hydroxide, and calcium 
hydroxide, respectively, to the four series to produce approximately 
the desired pH value before making each solution up to the re- 
quired volume. The pH values were finally determined at 20° C. 
by means of the hydrogen electrode and the solutions were analyzed 
according to the official method. The effect of the added acid 
of ne upon the per cent of insoluble matter found is shown in 

ig. 87. 

The solution receiving no addition of acid or alkali had a pH value 
of 4.60. As the pH value was lowered from this, by the addition 
of either sulfuric or hydrochloric acid, there was an increase in the 
per cent of insoluble matter found, sulfuric acid proving the more 
effective in causing precipitation. With increasing pH value, there 

* Effect of Hydrogen-Ion Concentration upon the Analysis of Vegetable Tanning Ma- 


terials. J. A. Wilson and E. J. Kern. J. Ind. Eng. Chem. 14 (1922), 1128. ; 
7J. Am, Leather Chem. Assoc. 16 (1921), 113. 


VEGETABLE TANNING MATERIALS 211 


was first a decrease in the amount of insoluble matter and the un- 
filtered solution gradually became more nearly transparent. In the 
case of the liquors containing sodium hydroxide, this continued with- 
out a break, the liquor having a pH 
value of 11.35 being quite trans- 
parent. But at the neutral point, an 
abrupt change occurred in the solu- 
tions containing calcium hydroxide; 
with further rise in pH value, the 
tannin was precipitated in increasing 
amounts. 

If these data may be applied 
quantitatively to raw tanning ma- 
terials in general, it is evident that 
the precipitation of tannin by lime 
may be prevented by keeping the 
pH value of water and liquor, dur- 
ing extraction, under 7. But to 
avoid appreciable oxidation effects, 
the material should not be ex- 
tracted at pH values greater than 
5, which may be accepted tentatively 


Added Acid Added Alkali 


- Original Extract) 


Insoluble Matter (Pero ent 


5 2 4 
as the optimum pH value for leach- pH Value of ‘Tan tiqnees 
ing, since, with decreasing values 
rie 98 pe ; 8 ' f Fic. 87.—Effect of pH value on per 
ere 1S an increasing amount o cent of insoluble matter in solution 


material precipitated. Where only of quebracho extract. 

hard water is available for leaching, 

it would seem the part of wisdom to add to it, before using, a sufficient 
quantity of acid to lower its pH value to 5. 


Clarifying, Decolorizing and Drying. 


In the manufacture of tanning extracts for sale, it is desirable 
that the extract should be clear, have a good color, and be dried to 
a degree sufficient to make handling and shipment easy.  Clarifica- 
tion, which consists merely of the removal of finely divided matter 
in suspension, is effected by settling and decantation, by filter-pressing, 
or by centrifuging. 

Where the extract manufacturer has carried the extraction of the 
raw material nearly to the limit, the extract is apt to have a dark 
color, which is-not desirable. This seems to be due to the extraction 
of foreign matters at the high temperatures used, or, in some cases to 
oxidation. A common method of clarifying and decolorizing some ex- 
tracts is bymmeans of blood albumin. The tan liquor is treated with 
a solution of blood albumin and then heated to a temperature of 
70° C., at which the albumin coagulates and carries down with it the 
suspended matters, some of the deeply colored bodies, and some tannin. 
The clear liquor is decanted off and the sludge is filter-pressed to re- 


212 THE CHEMISTRY OF LEATHER MANUFACTURE 


cover the adhering tan liquor. Although some tannin is lost in this 
way, the color of the extract is greatly improved. 

A number of other methods of decolorizing involve the treatment 
of the tan liquor with chemicals.. Sulfur dioxide and sodium bisul- 
fite are often used. Some brightening of the color would naturally 
be expected from the lowering of the pH value of the liquor by sul- 
fur dioxide, but the total effect seems to be more complex than this, | 
since some of the suspended and difficultly soluble matters are thereby 
rendered soluble. Apparently the reducing action of sulfur dioxide 
plays: aapart... 4 

There are naturally numerous methods in use for drying extracts. 
Since high temperatures and contact with the air during drying are 
undesirable, much of the drying is done in specially constructed vacuum 
dryers. As these have been greatly improved, from time to time, it 
has become possible to dry extracts to greater extents without causing 
them to suffer any damage. Formerly it was customary to reduce 
the water content of most extracts only to from 50 to 60 per cent, 
but now it is not uncommon to find extracts on the market having a 
water content as low as Io per cent. 


Chapter 12. 
The ‘Tannins. 


Some idea of the volume of literature which has appeared dealing 
with the composition of tanning materials may be gained from the 
bibliography, compiled by Dean,’ of the more important papers pub- 
lished prior to 1910, which lists 273 papers. It is remarkable that 
the greatest work on the organic chemistry of the tannins was accom- 
plished by the same man who did most to elucidate the complex struc- 
ture of the proteins, Emil Fischer. Among the numerous papers by 
Fischer and his coworkers, telling of their work which led to the 
discovery of the composition of tannin, may be mentioned one en- 
titled “Synthesis of Depsides, Lichen-Substances and Tannins,” 2 which 
is something in the nature of a review. The tannin studied by Fischer 
was that obtained from nutgalls, the so-called gallotannic acid and 
purest form of pyrogallol tannin. 

As early as 1852, Strecker * concluded that tannin was a compound 
of glucose and gallic acid. He was supported by the works of van 
Tieghem,* who found glucose among the hydrolytic products of tannin, 
and Pottevin,? who effected the hydrolysis with the enzyme of 
Aspergillus niger. But the variation in proportion of glucose found 
weakened the view, which gave way to that of Schiff,* who regarded 
tannin as digallic acid: 


Be ee OOF! 

Ga SCOOS. S 
ee oO 

HO COOH. 


Although Schiff’s formula for tannin was widely accepted, it was shown 
very definitely that digallic acid is not tannin. The formula showed 
no asymmetric carbon atom in the molecule to account for the optical 
activity of the natural tannin and it could not account for the high 
molecular weights observed. By observing the electrical conductivity, 
hight absorption, and behavior towards arsenic acid, Walden? showed 
that Schiff’s digallic acid is very different from natural tannin. » 
Fischer and Freudenberg * first set out to determine whether the 


'On the Composition of Taaning Materials; Bibliography 1828-19009. A. L. Dean. le 
Am. Leather Chem. Assoc. 6 (1911), 172. ; 

* Emil Fischer, Ber. 46 (1913), 3253. J. Am. Chem. Soc. 36 (1914), 1170, 

SA, Strecker. Ann. 81 (1852), 248; 90 (1854), 328. 

*P. van Tieghem. Annal, d. Sciences naturelles V. Serie Botanique (1867), 210. 

SH. Pottevin. Compt. rend. 132 (1901), 704. 

Pet ecu. er,-4 (1871), 232; 967; 12. (1879), 33. 

TP. Walden. Ber. 30 (1897), 3151; 31 (1898), 3167. 

8 E. Fischer and K. Freudenberg. Ber. 45 (1912), 919. 


213 


214 THE CHEMISTRY OF LEATHER MANUFACTURE 


glucose found by Strecker was really a constituent or only a chance 
impurity of tannin. They started with the purest technical tannin 
available. Assuming that the tannin molecule had no carboxyl group, 
they proceeded to separate it from acid impurities by rendering its 
solution slightly alkaline and extracting it with ethyl acetate, a method 
discovered independently and published previously by Paniker and 
Stiasny.° As they had anticipated, the tannin dissolved in the ethyl 
acetate, leaving the sodium gallate in the aqueous solution. They 
accepted this as proof that the tannin possessed no free carboxyl group. 
Applying this method of purification to different kinds of commercial 
tannin, they obtained products that were practically identical. 

After hydrolyzing the purified tannin with sulfuric acid, they 
found between 7 and 8 per cent of glucose. In the purest sample 
of tannin examined, they found one molecule of glucose combined 
with ten molecules of gallic acid. No phenolcarboxylic acid other 
than gallic could be found in tannin, even when the hydrolysis was 
effected by means of alkali. With excess of alkali and exclusion of air, 
large yields of alkali salt of gallic acid were obtained in relatively 
pure condition. 

It appeared to Fischer that the surest way to prove his assump- 
tions regarding the structure of tannin was to synthesize it. He started 
out with the idea that tannin contains no carboxyl and that, conse- 
quently, the gallic acid must all be bound as an ester, a condition that 
would be fulfilled by regarding tannin as an ester-like combination 
of one molecule of glucose with five molecules of digallic acid, after 
the manner of pentacetyl glucose. 

The investigations of Fischer and his collaborators are so extensive 
as to require treatment in a separate volume and the reader is referred 
to the recent book by Freudenberg,’® who is continuing Fischer’s work 
on the tannins. Fischer ‘* succeeded in preparing penta-m-digalloyl- 
B-glucose, which was proved to be an isomer of the tannin from 
Chinese nutgalls. The formula for the so-called gallotannic acid may 
thus be written 


NE ce eee NG hE ‘ 
ie emie der Nattirlichen Gerbstoffe. K. Freudenberg. J, Springer, B li 
1 E, Fischer and M. Bergmann. Ber, 51 (1918), 1760. 8 Je Spree (1920). 


THE TANNINS 215 


Freudenberg has suggested a classification of the tannins more dis- 
tinctive than the catechol-pyrogallol system mentioned in Chapter 11. 
He would divide them into two main classes, the first consisting of 
hydrolyzable tannins in which the benzene nuclei are united to larger 
complexes through oxygen atoms, and the second of condensed tannins, 
in which the nuclei are held together through carbon linkages. Where 
both kinds of compounds are present, as in ellagic acid, the classifica- 
tion is decided by the genetic connection with other tannins. 

The first group embraces three classes: (1) depsides, esters of 
phenolycarboxylic acids with each other or with other oxyacids; (2) the 
tannin class, or esters of phenolcarboxylic acids with polyatomic alco- 
hols and sugars; and (3) glucosides. The most important criterion 
of the first group is hydrolysis to simple components by enzymes, 
particularly tannase or emulsin. Freudenberg and Vollbrecht?? have 
recently discussed the isolation and determination of the activity of 
tannase, which is secreted by Aspergillus niger. 

The second group of tannins are not decomposed to simple com- 
ponents by enzymes. They are generally, but not always, precipitable 
by bromine and condense to amorphous tannins, or reds, of high 
molecular weight, when treated with oxidizing agents or with strong 
acids. They are divided into two classes according to whether or not 
phloroglucin is present. With the exception of some simpler ketones, 
oxybenzophenones and oxyphenylstyrylketones, the catechins belong 
to the phloroglucin class, which include the tannins of quebracho and 
probably also those of oak bark. 

That the reds, or phlobaphenes, precipitated by acid from solu- 
tions of quebracho and gambier extracts are oxidation products is 
indicated by the curves in Fig. 86, which show that the quantity of 
precipitate obtained is greatly increased by previous oxidation. The 
actual composition of the phlobaphenes is not yet known. The ellagic 
acid, or bloom, formed in solutions of tanning extracts of the 
pyrogallol group, is of very much simpler composition than the 
phlobaphenes. The formula 


aN sa 
wk Jo.co-h z OH 


for ellagic acid, suggested by Graeb,'* is the most satisfactory thus far 
proposed. 


Practical Definition of Tannin. 


The great classical work on the structure of the tannins is still 
too far from complete to enable one to apply organic chemistry to 
practical tanning, excepting, perhaps, in the study of the reactions 


12 Z. physiol. Chem, 116 (1921), 277. 
1% Chem, Ztg. (1903), 129. 


216 THE CHEMISTRY OF LEATHER MANUFACTURE 


of particular groups present in the tannin molecules. The struc- 
tures of the tannins of the catechol group are still entirely unknown. 
As in the case of the proteins, it has been found necessary to deal 
with the general properties of the tannins from the standpoint of 
physical rather than organic chemistry. 

All tannins seem to have the property of precipitating gelatin from 
solution and of combining with the protein matter of hide. fibers, 
forming a compound resistant to washing. Any natural vegetable 
material having this property in aqueous solution has generally been 
accepted as tannin, and this has been made the basis for the various 
methods of determining tannin now in use. The portion of soluble 
matter which neither combines with collagen to form a compound 
resistant to washing nor precipitates gelatin from solution is known as 
nontannin, 5 


The Gelatin-Salt Test for Tannin. 


In testing a solution for the presence of tannin, it is customary 
to add to it one drop of a solution made. by dissolving 10 grams of 
gelatin and 100 grams of sodium chloride in a liter of water, a precipi- 
tate or turbidity indicating the presence of tannin. This reaction has 
been the subject of numerous investigations for more than a century. 
Its sensitivity as a means of detecting tannin in solution has recently 
been studied by Thomas and Frieden.1* They found that the added 
gelatin is completely precipitated when the ratio of gelatin to tannin 
does not exceed 0.5; a great excess of gelatin prevents precipitation. 

Thomas and Frieden studied the precipitation of tannin by gelatin 
at different pH values and concentrations of salt. Using a gelatin 
solution containing no salt, they obtained a maximum precipitation of 
gallotannic acid, in pure solution, at a pH value of 4.4; at pH values 
below 4 or above 5, the solutions became opalescent, but no precipitate 
formed. The effect of adding sodium chloride was to widen the range 
of pH value over which a precipitate was obtainable: it apparently 
had no effect upon the sensitivity of the test between the pH values 4 
and 5. Using various commercial tanning extracts, they found that 
the optimum range for precipitation of tannin by gelatin varied from 
3-5 to 4.5, quebracho, wattle, and hemlock precipitating most readily at 
pH values slightly above 4.0 and gambier, oak, and larch at values 
slightly below 4.0. 

The limits of dilution at which tannin could be detected by means 
of the gelatin-salt reagent were found to depend upon the proximity 
of the solution to the optimum pH value for precipitation, which 
is different for each kind of extract, but apparently always lies between 
3-5 and 4.5. At the optimum pH value, gambier, the least sensitive 
to the test, could be detected at a concentration of 1 part of tannin 
to I10,000 parts of water. Wattle, the other extreme, could be de- 
tected at a dilution of 1 to 200,000. When the commercial extracts 


** The Gelatin-Tannin Reaction. A. W. Thomas and A. Frieden. Ind. Eng. Chem 
(1923); (advance copy). ‘ : 


THE TANNINS 217 


were simply diluted with distilled water, no attention being paid 
to the final pH values, the sensitivity of the tests was greatly de- 
creased. ‘The least sensitive was then hemlock at 1 part in 6,500 and 
the most sensitive was gambier at I part in 30,000. They also found 
that the age of the gelatin-salt reagent has no effect on the sensitivity 
of the test, provided bacterial action is prevented by means of toluene. 


The Determination of Tannin. 


Although a general discussion of analytic methods is outside the 
scope of this book, the question of determining tannin demands some 
attention here because of its importance in leather chemistry and the 
fact that the methods in common use do not determine the actual tannin 
content of tanning materials, but include as tannin a variable fraction 
of nontannin, which, in the extreme case of gambier, is twice as great 
as the tannin content itself. 

For more than a century leather chemists have struggled with the 
question of determining tannin and numerous methods have been 
proposed. Of these, the only one which has really survived is that 
known as the hide powder method. But even this is used in different 
parts of the world with different modifications. For a review of the 
various methods proposed up to 1908, the reader is referred to Procter’s 
book.*® It will serve our purpose here to give an outline of the 
official method of the American Leather Chemists Association, which 
is similar in principle, although not in all details, to those employed 
in various parts of Europe. 


A. L. C. A. Method '* 


“American Standard” hide powder is specially prepared by giving it 
a light tannage with chrome alum, washing it practically free from 
soluble matter, and squeezing it until it contains not less than 71 nor 
more than 74 per cent of water. The solution of tanning material 
for analysis must contain not less than 0.375 nor more than 0.425 
gram of tannin per 100 cubic centimeters, as found by this method. 
To 200 cubic centimeters of this solution is added such an amount 
of the wet hide powder as contains not less than 12.2 nor more than 
12.8 grams of dry hide powder and the whole is shaken for 10 minutes. 
The detannized solution is separated from the powder by squeezing 
through linen and is then filtered through paper, after the addition 
of kaolin, the solution being returned to the paper until the filtrate 
is quite clear. The amount of residue from an aliquot portion of this 
filtrate, after correcting for the water introduced by the hide powder, 
is taken as a measure of the nontannin in the original material. The 
difference between the total soluble matter and the nontannin is called 


#® Leather Industries Laboratory Book. H, R. Procter. E. & F.N. Spon, London (1908). 
46 For further details, see J. Am. Leather Chem. Assoc. 16 Ci9Z2T), Tres: 


218 THE CHEMISTRY OF LEATHER MANUFACTURE 


tannin. ‘The other determinations of the method need not concern 
us here. 3 

It will be apparent from the discussion of the equilibria of pro- 
tein systems in Chapter 5 that the method involves two false as- 
sumptions: one that the hide powder combines only with tannin; 
the other that the solution absorbed by the collagen jelly has the same 
concentration as that in the surrounding solution. It may be men- 
tioned that the former assumption introduces errors vastly greater 
than the latter. As long ago as 1903, Procter and Blockey 1” showed 


TABLE XVII. 


RESULTS OF TREATMENT OF PuRE GALLIC Acrtp Sotutions By A. L. C. A. Metuop. 


(Using 47 grams of wet hide powder (73 per cent water) to 200 c.c. of solution.) 


Gallic Acid Nontannin Tannin 
Grams per liter Per cent Per cent 
8.88 54.0 46.0 
4.44 47.1 52.9 
2.22 43.8 56.2 
EG 40.4 59.6 


TABLE XVUL 


EFFECT OF ALTERING ProporTION oF Hinze Powper upon AMountT oF GALLIC AcIp 
REMOVED FROM A 0.888-PER CENT SOLUTION. 


(Using principle of A. L. C. A. method.) 
Wet Hide Powder 


(73 per cent water) Nontannin Tannin 
Grams per 200 C.c. Per cent Per cent 
5 91.8 8.2 
10 86.0 14.0 
25 69.6 30.4 
50 52.1 47.9 
75 43.7 56.3 


that hide powder removes from solution considerable amounts of such 
nontannins as gallic acid, quinol, and catechol. Wilson and Kern 18 
showed this even more strikingly by subjecting pure solutions of 
gallic acid to the A.L.C.A. method of tannin analysis. By varying 
the concentration or the proportion of hide powder, practically any 
results desired could be obtained. Tables XVII and XVIII show 
that the A.L.C.A. value for tannin decreases with increasing concen- 
tration of the solution and increases with the proportion of hide 
powder. Using a solution of I gram of gallic acid per liter, the 
method indicates a tannin content for the sample of about 60 per 
cent, even though it contains none at all. 


_™ Absorption of Non-Tanning Substances by Hide Powder and Its Influence on the 
Seana: of Tannin. H. R. Procter and F. A, Blockey. J. Soc. Chem. Ind. 22 (1903), 


2 
7®The Nontannin Enigma. J. A. Wilson and E. J. Kern. J. Am. Leather Chem. Assoc. 
13 (1918), 429. 


Pa hei an NSS 219 


Wilson-Kern Method. 


With the object of avoiding the palpable errors of the A.L.C.A. 
method, Wilson and Kern !® set out to devise a method that would 
determine exactly what is called for in the practical definition of 
tannin, namely, that portion of the soluble matter of vegetable tanning 
materials which will precipitate gelatin from solution and which will 
form compounds with hide fiber which are resistant to washing. The 
principle of their method is to shake a convenient amount of the 
tannin solution with a known quantity of purified hide powder until 
all tannin has been removed from solution, as determined by the 
gelatin-salt test. The tanned powder is then washed free from soluble 
matter including the nontannin removed from solution by the hide 
powder, which is responsible for the large errors in the A.L.C.A. 
method. It is then carefully dried and analyzed for tannin as in the 
regular procedure for vegetable-tanned leathers, and from this figure 
the per cent of tannin in the original material may readily be calculated. 

In order to show the workability of this method, Wilson and Kern 
selected 8 typical tanning materials showing great differences in prop- 
erties, especially in so-called astringency.. The solid quebracho ex- 
tract and the four liquid extracts of oak bark, larch bark, chestnut 
wood, and osage orange are typical samples of the best of these ma- 
terials on the American market. The gambier is the ordinary pasty 
product from the East Indies; the sumac, consisting of ground leaves 
and small twigs, is from Palermo; and the hemlock bark came from 
the forests of Wisconsin. The extracts were simply dissolved in hot 
water, cooled slowly, and made up to the mark. The bark and sumac 
were finely ground and leached by percolation, only the extracted por- 
tions being used after making up to a definite volume. In each teser ie 
grams of hide powder (of known hide substance content) were put 
into a wide-mouthed, rubber-stoppered, half-pint bottle, the tanning ma- 
terial dissolved in 200 cubic centimeters of solution was added, and 
the whole was shaken in a rotating box for 6 hours. 

The amount of material that could be used was limited by the 
amount of tannin that the hide powder was capable of taking up in 
6 hours. On the other hand it was desirable not to use too little, 
since the less the amount of tannin fixed per unit of hide sub- 
stance,. the less the accuracy of the method, since the tannin was 
determined by difference. Whenever the liquor, after the 6-hour shak- 
ing, gave a turbidity or precipitate with the gelatin-salt reagent, the 
test was repeated with less material. 

The tanned powder was washed by shaking with 200 cubic centi- 
meters of water for 30 minutes, squeezing through linen, and re- 
peating the washing operation until the wash water showed no color 
and gave no test with ferric chloride solution. Nontannins like gallic 
acid give a dark coloration upon the addition of ferric chloride solu- 
tion. Except for the osage orange and chestnut wood extracts, which 


% The True Tanning Value of Vegetable Tanning Materials. J, A. Wil ; 
Kern. J. Ind. Eng. Chem, 12 (1920), 465. J Met. 


B20, THE CHEMISTRY OF LEATHER MANUFACTURE 


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THE TANNINS 221 


are unusual in several respects, not more than 12 washings were re- 
quired to free the powders from nontannin, which shows that the line 
of demarcation between tannin and nontannin is fairly sharp for the 
commoner materials. The wash water continued to extract coloring 
matter from the powders tanned with osage orange until after the 
fiftieth washing, while as many as 25 washings were required to free 
the powders tanned with chestnut wood from soluble matter producing 
a dark color with ferric chloride. All wash water was tested with the 
gelatin-salt reagent, but in every case the test was negative. 

The washed powders were dried at room temperature for 24 hours 
or longer and then analyzed for water, ash, fat and hide substance. 
The per cent of hide substance was taken as the per cent of nitrogen 
multiplied by 5.62. The difference between 100 and the sum Oietie 
percentages of water, ash, fat and hide substance was taken as the 
per cent of tannin in the tanned powder. The parts of tannin per 
100 parts of hide substance divided by the parts of tanning material 
used per part of hide substance gave the per cent of tannin in the 
original material. The results for the 8 materials examined are given 
in triplicate in Table XIX. 


Comparison of A. L. C. A. and Wilson-Kern Methods. 


The two methods just described give very different results. A 
careful comparison is therefore desirable, especially since it will assist 
in giving a better understanding of the vegetable tanning process and 
of what is ordinarily called tannin. It should be remembered that the 
great majority of tannin values quoted in the literature were obtained 
either by the A.L.C.A. method or by some method based upon similar 
principles. The Wilson-Kern method is still too new to have found 
general acceptance. 


TABLE XX. 

Percentage Analysis of Material Per- 

Wilson- centage 

A. L. C. A. Method Kern Error in 

Insoluble Soluble Matter Method A.L.C.A 

Material Water Matter Nontannin Tannin Tannin Method 
CHeBIACHO 55... 17.87 ma6 6.96 68.01 47.41 43 
Hemlock Bark.... . 8.90 74:33 6.71 10.00 6.17 63 
De a Sie rn 52.66 3.68 19.46 24.20 12.88 88 
Panenusatk...<s = 51.08 5.88 20.90 22.14 11.71 89 
Chestnut Wood.... 58.90 1.50 13.80 25.80 11.90 117 
COME Eat wee ss 5's 9.25 47.20 17.990 25.50 9.61 166 
Osage Orange..... 46.05 3.45 10.63 39.87 13:37 198 
SEES eee eeore 5.30 18.57 24.95 7.79 220 


For the sake of comparison, Wilson and Kern analyzed_ the 8 
materials they studied by both methods and the results are given in 
Table XX. The percentage error in the A.L.C.A. method is cal- 
culated on the assumption that the results of the Wilson-Kern method 
are correct. Although the enormous errors in the A,L.C.A. method 


222 THE CHEMISTRY OF LEATHER MANUFACTURE 


LADLE xO 


EFFECT OF VARIATION IN Amount oF Hing Powper Usep upon Per CENT OF 
TANNIN OBTAINED By A. L. C. A. MEtHop. 


Wet Hide Powder 
(73 per cent Apparent Percentage 
water) Used to Per cent of Error 
Detannize 200 cc. Tannin by Due to 
Grams Tan Liquor ALL IGaeS Ant Cas 
Material per Liter Grams Method Method 
ONERERCHO © eos wee 3 93.3 68.18 44 
46.7 67.56 43 
26.7 66.61 40 
13.3 64.36 36 
6.7 57.50 21 
Hemlock “Baris 8 20 03.3 10.98 78 
40.7 10.60 G2 
26.7 9.76 58 
13.3 9.35 52 
6.7 7.98 29 
OE SR B5 dan ORE nae Asty 93.3 25.02 04 
46.7 24.59 gi 
23:8 24.01 86 
11.7 22.09 72 
5.9 18.77 46 
Larch mark. 5 view ok 4.37 03.3 28.10 140 
46.7 24.52 109 
23.3 21.97 88 
11.7 19.10 63 
5-9 16.24 39 
Chestnut Wood ...... 15 03.3 26.87 126 
46.7 25.80 EIy 
26.7 24.59 107 
23.3 23.52 98 
16. 22.49 — 89 
SISTA LRA de ate elie 4 03.3 24.98 160 
46.7 25.05 161 
23.3 24.47 155 
11.7 23.45 144 
5.9 21.45 123 
Osage Orange 222.7. 8 93.3 . 40.48 203 
46.7 39.47 195 
26.7 38.21 186 
13.3 36.27 174 
9.3 35-67 167 
Gambier eee 4.58 03.3 29.04 273 
46.7 25.60 229 
a8 22.50 190 
liz 17.22 121 


5.9 13.38 72 


THE TANNINS 223 


are nothing short of sensational, they are probably not at all exag- 
gerated. But the extent of these errors is less surprising in view of 
the large proportion of such nontannins as gallic acid that appear as 
tannin by the A.L.C.A. method, as shown in Tables XVII and XVIII. 

The need for arbitrary limits in the A.L.C.A. method was clearly 
shown by the gallic acid experiments, but was more strongly em- 
phasized by similar experiments upon actual tan liquors. The effect of 
altering the proportion of hide powder with solutions of the 8 tanning 
materials is shown in Table XXI and in Figs. 88, 89, and 90. In Figs. 


“ CHESTNUT WOOD EXTRACT 


OSAGE ORANGE EXTRACT 


OAK BARK EXTRACT 


SICILIAN SUMAC LEAVES 


Apparent Percent, of Tannin in Samvle 
Apparert Percent, of Tannin in Sample 


LARCH BARK EXTRACT 15 GAMBIER EXTRACT 


20 40 60 80 20 40 60 80 


Grams Wet Hide Powder 
per 200 c.,c, Tan Liquor 


Fic. 88.—Effect of variation in 
amount of hide powder used upon 
the determination of tannin by 


Grams Wet Hide Powder 
per 200 c.c,. Tan Liquor 


Fic. 89.—Effect of variation in 
amount of hide powder used upon 
the determination of tannin by 


A.L.C.A. method. Ac LG] A; method. 


88 and 8&9 the short, vertical lines are placed at points on the curves 
corresponding to the smallest amount of hide powder that would 
completely detannize the solutions under the conditions of the A.L.C.A. 
method, as determined by the gelatin-salt test. The crosses indicate 
the points corresponding to the quantity of hide powder called for 
in the A.L.C.A. method. The zero points represent the percentages 
of tannin found by the Wilson-Kern method and the broken portions 
of the curves are extrapolated. 

No scientific reason has ever been given for the selection of the 
particular amount of hide powder called for in the A.L.C.A. method. 
So far as the principle of the method is concerned, any of the values 
given in Table X XI might be accepted as correct, since the solutions 
were completely detannized in every case. ‘This should be borne in 


224 THE CHEMISTRY OF LEATHER MANUFACTURE 


mind when employing figures for tannin appearing in the literature, 
which have been found by this or similar methods. 

As would be expected, the greatest errors in the A.L.C.A. method 
are obtained with those materials 
containing the greatest proportion 


terials which give the least errors 
by the A.L.C.A. method are most 
20 40 60 80 astringent, while those giving great- 
seer cee Ape est errors are least astringent. The 

a pserabrie a order of the materials in Table XX 
Fic. 900.—Effect of variation in amount might almost be taken as the order 
of hide powder used upon the error fdectees . Ith * 
involved in determining tannin by 2! Gecreasing stringency, althoug 
the A.L.C.A. method. an exact parallelism cannot be 
claimed. Quebracho and hemlock 

bark are generally conceded to be the most astringent and sumac and 
gambier the least astringent of these materials. This suggested a relation 


5 

dt 

@ ps of nontannin to tannin. Quebracho, 
z a. having least nontannin, gives the 
2 smallest error. However, if the que- 
ee, bracho is mixed with gallic acid to 
x 200 make the proportion of nontannin 
5 4160 to tannin about the same as in the 
ex 160 case of the gambier, it gives errors 
& 140 nearly as great as in the case of the 
# 120 gambier, which is shown in Table 
100 XXII. 

- 80 Comparison of the two methods 
3 60 has brought out at least one fact of 
# 40 practical significance: Those ma- 
© 

é 


0% 
(eo) 


TABLA A XSL 


MIXTURE OF QueBRACHO Extracr AND GaLuic Aci. 


Wet Hide 
Powder Percentage Analysis of Mixture of 5 
(73 per Parts of Quebracho Extract to 9 Percentage Error in 
cent water) Parts of Dry Gallic Acid A. L. C. A. Method 
Used to 
Detannize Wilson- In Pres- 
200 Cc. A.L. C. A. Method Kern Alone ence of 
Tan Liquor Insoluble Soluble Matter Method (from Gallic Gambier 
Grams Water Matter Nontannin Tannin Tannin Fig.90) Acid Alone 
DSigunecs 5.80 3.96 33-87 56.37 16.93 44 233 273 
CaS a a 5.80 3.06 37.14 53.10 16.93 43 214 229 
oh Pn aap 5 80 3.06 44.07 46.17 16.93 39 173 190 
ete eG ee eae 3.90 53.39 36.85 16.93 33 118 121 
Sie es 5.80 3.96 63.34 26.90 16.93 -18 59 72 


between astringency and the ratio of nontannin to tannin. Astringency 
appears to be a function of the rate of combination of tannin and 
protein, In the experiments listed in Table XIX, the hide powder 


THE TANNINS 226 


fixed more than twice as much tannin from the quebracho liquors in 
3 hours as from the gambier liquors in 6 hours. But, when enough 
gallic acid was added to the stronger quebracho liquors to give them 
the same ratio of nontannin to tannin as in the gambier, the hide 
powder did not remove anywhere nearly all the tannin in 6 hours. 
Upon addition of the gelatin-salt reagent to the liquors after the 
6-hour shaking, huge precipitates were formed, suggesting a great re- 
duction in astringency. That the effect was only one of slowing’ 
up the tanning action was proved by the fact that the hide powder was 
able to detannize the solution completely in 24 hours. This also ex- 
plains the mild action of tan liquors which have been used a great 
many times and have consequently accumulated a large amount of 
nontannin. 

The polemics following the publication of the Wilson-Kern method 
served to stimulate investigations of the properties of tanning ma- 
terials. At the 17th annual meeting of the American Leather Chemists 
Association a formal discussion 7° of the Wilson-Kern method was 
staged, and the chief aim of the opposition was apparently to show 
that the low results obtained were due to losses of tannin in the manipu- 
lation. It was contended that a certain proportion of the tannin of a 
liquor will form a stable compound with hide only after long contact, 
and, further, that even tannin which has already combined with the 
hide powder will be removed to an appreciable extent during the wash- 
ing required by the Wilson-Kern method, but no substantial evidence 
was offered in support of these contentions: 


Effect of Washing. 


Certain differences in behavior of the several different tanning 
materials have caused a widespread belief that some tannins form 
more stable compounds with skin that others; for example, the tannin 
from gambier is supposed to form a compound with skin less stable 
than that from hemlock bark. It has also been supposed that mixtures 
of tanning materials behave differently in this respect from the 
individual materials. 

Wilson and Kern ** made a careful study of the possible losses of 
tannin during the washing operation involved in their method and came 
to the conclusion that any such loss was too small to have any effect 
upon the determination. Table XXIII shows that practically the same 
results are obtained for a great variety of tanning materials, whether 
the tanned hide powders were washed 15, 25, or 50 times. Theoretically, 
tanning may be reversible, but the rate of hydrolysis is so small as 
to have no bearing on the Wilson-Kern method, which holds equally 
well for both mild and astringent tanning materials. 


20 Printed in full, J. Am. Leather Chem. Assoc. 15 (1920), 451. 
71 Nature of the Hide-Tannin Compound and Its Bearing upon Tannin Analysis. J. A. 
Wilson and E, J. Kern. J. Ind. Eng. Chem, 12 (1920), 1149. 


226 THE CHEMISTRY OF LEATHER MANUFAC URS 


TABLE XXIII. 


EFFECT OF EXcESSIVE WASHING OF THE TANNED HinE PowpEerR UPON THE PER 
CENT oF TANNIN FounpD BY THE WiLSON-KERN METHOop. 


Hide 
Substance 
in Powder Per cent Tannin in 
Used to Extract. Value Obtained 
Extract Detannize from Analysis of 
Gramsin 200 cc. Tanned Powder Washed 
2zoocc. Solution 15 25 50 
Extract Solution Grams Times Times Times 
OuebhrachOse a sche e Sens ee ae ee 3.80 10.44 46.84 47.25 46.90 
Graber. We ee ee ee oor 10.00 10.44 7.87 7.89 7.67 
Gambier-quebracho mixture*.... 6.90 10.44 20.67 20.34 20.43 
chestrintewood ste crate bane. 13.60 10.32 tee 13.99 13.93 
Hemlock sparke.u.e sees eee 13.00 10.32 23.47 23.38 23.50 
Chestnut wood - hemlock bark 
AUSTUIPO ra ae eee 13.30 10.32 alee 18.73 19.05 
Oaksbark ao. t ee ee oe 13.60 10.40 15.52 15.36 15.35 
Barchiibar’ fi0cs sien? ost Sete Oe 10.32 —.t 11.29 _ 11.28 
ITH AC yen PE ee chen haa ie 13.00 10.39 16.36 16.29 16.39 
Wattles bark couicrcta cere eee nie 8.00 10.32 24.66 24.16 24.73 


* Mixture of 19 parts solid quebracho extract to 50 of gambier extract. 

t Mixture of 68 parts of chestnut wood extract to 65 of hemlock bark extract. _ : 
i + Lalepiation not made because 15th wash water gave test for nontannin with ferric 
chloride. 


Conversion of Nontannin into Tannin. 


In criticizing the Wilson-Kern method, Schultz 2 said, “We have 
taken the nontannins and washings and reconcentrated them under a 
high vacuum to the original volume of 200 cc. and have tanned hide 
powder with it, and, by the calculations employed, we have found 
a definite percentage of tannin.” He mentioned also that the con- 
centrated liquor gave a positive test for tannin with the gelatin-salt 
reagent. It might look at first sight as though the detannized liquor 
and wash waters, before concentrating, really had contained tannin 
and Schultz evidently so regarded it. Wilson and Kern confirmed 
Schultz’s experimental finding while analyzing a sample of gambier _ 
extract by their new method. The detannized liquor and 15 wash 
waters, all of which gave no test with the gelatin-salt reagent, were 
concentrated to 200 cubic centimeters, whereupon they were found to 
give a bulky precipitate with the reagent. But, when diluted back 
to 3200 cubic centimeters, they still gave a bulky precipitate with the 
gelatin-salt reagent, showing that a most Important chemical change 
had taken place during the concentrating. 

Another sample of gambier was analyzed by the new method and 
found to contain 7.94 per cent of tannin. The detannized liquor and 
17 wash waters, 3600 cubic centimeters in all, were evaporated to 250 
cubic centimeters, analyzed by the new method, and found to contain 


2 (Discussion), J. Am. Leather Chem, Assoc, 15 (1920), 455, 


THE TANNINS 227 


5.56 parts of tannin per 100 of original extract, giving the extract a 
total of 13.50 per cent tannin. The detailed results are given in 
Table XXIV. i 
PABEE XXIV; 


GAMBIER EXTRACT. 


Two hundred cubic centimeters of solution containing 9.00 grams of extract 
were detannized with 12 grams of air-dry hide powder, containing 10.40 grams of 
hide substance, and then the tanned powder was washed 17 times with a total of 
3400 cubic centimeters of water. The residual liquor and wash waters were evapo- 
rated to 250 cubic centimeters and used to tan 12 grams of fresh hide powder, 
which was afterwards washed as usual. 

Hide Powder Tanned in 


Original Concentrated 
Analysis of Air-Dry, Tanned Powder Solution Wash Waters 
SS 17.31 16.24 
a ee ra cer ah Ol Ges 0.14 
Pee eloratorm extract)... ..<..c.s<ecseccec cs 0.39 0.42 
Boeeevearetance (N x 5.62). ........0isceecece 76.86 79.38 
rer SCINETETICE) oo... kas ccc ecco ccs 5.28 3.82 
Per 100 grams hide substance: 
permet TATS 4) Fs. yess cad os ccc e'e 6.87 4.81 
Pereeiabe Sed, OTAMS.. 2... cece cece eccs 86.54 86.54 
Peter tainitt in extract... ....0..esesese: e704 5.56 


Total tannin, either originally present or formed during the concentrating 
of the wash waters, 13.50 per cent. 


In order to show that this increased amount of tannin would have 
combined with the hide powder had it been present in the original 
solution, Wilson and Kern made up a new solution of this extract, 
concentrated and diluted back several times, and then analyzed it by 
the new method, finding 12.69 per cent tannin. If the concentrating 
had been continued a little longer, the figure 13.50 would probably 
have been reached or passed. The results are shown in Table XXV. 


TABLE XXV, 


GAMBIER EXTRACT. 
(Same as noted in Table XXIV.) 


Dissolved 60.00 grams of extract in 1 liter of water. Concentrated to 250 
cubic centimeters and diluted back to 1 liter. Repeated 3 times, the fourth time 
diluting to 2 liters. Two hundred cubic centimeters of diluted solution, containing 
6.00 grams of original extract, were detannized with 12 grams of air-dry hide 
powder containing 10.37 grams of hide substance, which was afterwards washed 
as usual. 

ANALYSIS OF AIR-DRY TANNED POWDER. 


De i 2S ish os ge eo ewe 18.23 
ale aah gl a Se ee a Un 0.18 
Peelerororm extract } iis eescies Shas koe. e. 0.42 
eieuvstance (Ni x5.62)......,..02e. dilou. Le. 75.62 
Memmnttes by dillerence) <6. cs os os odo ceek cnc ak 5.55 
Per 100 grams hide substance: 

Pernineiound,.¢rams— sy. 5s ose Lak wok wad 

Pitesial used: pranis (2.5, 5 26a eee a 57.86 


228 THE CHEMISTRY OF LEATHER MANUFACTURE 


In spite of the great change in the tan liquor produced by con- 
centrating, it is not shown to any appreciable extent in the analyses 
by the A.L.C.A. method shown in Table XXVI. Concentrating the 
tan liquor and diluting back caused a rise in per cent of tannin by 
the new method from 7.94 to 12.69, but the rise in the A.L.C.A. 
method is only from 26.14 to 26.40, which difference is so small as 
even to be attributable to experimental error. The reason for this 
small difference is probably that the nontannins which are convertible 
into tannin all combine with the hide powder initially, even though 
they are easily removed later by washing. ¢ 


TABLE XXVI. 


GAMBIER EXTRACT. 
(Same as noted in Table XXIV.) 


Both the original liquor noted in Table XXIV and the specially treated 
liquor noted in Table XXV were appropriately diluted and analyzed by the 
A. L. C. A. method. 


Per Cent of Original Extract 


Original Treated 

Liquor Liquor 
insolublesmatter ss Aco: sear 7.66 8.62 
Nonatantitt -. .50.. eee ee 18.33 17.57 
SLANT. civic kon he corset eee 26.14 26.40 


Just what chemical actions are involved in the conversion of non- 
tannin to tannin must remain a matter for speculation until more ~ 
data are available; oxidation, condensation, and polymerization may 
all be involved. It is conceivable that gallic acid might be converted 
into digallic acid under suitable conditions, and it seems extremely 
likely that a polymerized form of digallic acid would have tanning 
properties. A solution of pure gallic acid gives no test for tannin, 
but Wilson and Kern found that after boiling for some time it gives 
a bulky precipitate with the gelatin-salt reagent, and will then ap- 
parently tan skin. A detannized solution, which gives no test for 
tannin, can be made to give a strong test merely by passing oxygen 
gas through it. Long exposure to air has a similar action. It is evi- 
dent that the Wilson-Kern method furnishes a valuable means of 
studying the conversion of nontannin into tannin, and might conceivably 
be applied to a study of the formation of tannin in nature and to the 
aging of barks. : 


Effect of Aging. 


The conversion of nontannin into tannin is apparently responsible 
for two factors of great importance to tanners of heavy leathers, 
namely, the time factor in tanning and the aging of leather. In 
the A.L.C.A. discussion referred to, Alsop ?* remarked that sole leather 
tanned slowly not only contains more tannin, but actually consumes 


23 Toc. cit., p. 464. 


THE TANNINS 229 


less tanning material than the rapid tannages. In a private communi- 
cation to the author, H. R. Procter has called attention to the fact that 
leather stored for a long time, or aged, before washing contains more 
tannin than if it had been washed immediately after tanning. A 
number of critics have said that the Wilson-Kern method is weak 
because it does not include as tannin all of the material that can be 
made to combine with hide substance by aging. This argument, how- 
ever, is weak because the Wilson-Kern method offers a very satis- 
factory means of studying the aging properties of different tanning 
materials. An application of the method to such a study is shown 
in Table XX VII for ten commercial extracts or mixtures thereof. 


TABLE XXAVIL, 
Errect or AGING UPON PER CENT oF COMBINED TANNIN IN LEATHER. 


Three 12-gram portions of hide powder were used to detannize 200 cubic 
centimeters each of the solutions of tanning materials noted in Table XXIII. 
One portion in each case was washed 25 times immediately after tanning and 
the other two were allowed to dry without washing. Of these one was kept 
exactly 30 days and then washed 25 times; the other was kept just 1 year and 
then washed 25 times. 

Tannin as Per Cent of Original Extract 
By Wilson-Kern Method 


In In In 
Leather Leather Leather 
Washed Kept Kept 
Immediately 30 Days 1 Year By 
after before before * A.L.C. A. 
Extract Tanning Washing Washing Method 
aslo ees sven se es eee 47.25 53.00 54.50 60.87 
A 7.80 10.49 14.13 25.61 
Gambier-quebracho mixture.......... 20.34 23.92 QE 3A, epee Stee 
SE ols er 13.09 18.02 18.36 25.70 
Oe ES 23.38 24.87 25.46 26.68 
Chestnut wood-hemlock bark mixture 18.73 20.45 21.25 25.64 
6 0 | Oe, 15.30 17.23 20.08 26.19 
rg ek ows as os oeen esos 11.29 13.22 18.73 22.96 
MERRIE, Os arose ks coc cee ees 16.29 17.94 17.90 25.51 
eSMVAT Po ole dass cise cease ues 24.16 25.80 26.61 33.55 


It is interesting to note that in no case does aging for an en- 
tire year raise the tannin value to a point as high as that given by the 
A.L.C.A. method. Aging the gambier-tanned powder for a year raised 
the tannin content to about the same value as is produced by merely 
concentrating the liquor before tanning, as indicated in Tables XXIV 
and XXV. The change taking place upon aging is probably of the 
same nature as that described above as the conversion of nontannin 
into tannin. 

In the manufacture of vegetable-tanned upper leather, the effect 
of aging is probably not very marked. In actual practice, Wilson and 
Kern found barely 50 per cent as much tannin in the leather coming 
from a certain upper leather yard during a 3-year period as was put 
into it, according to the analyses by the A.L.C.A. method of the 


230 THE CHEMISTRY OF LEATHER MANUFACTURE 


extracts used. About half of the tannin seemed to be mysteriously 
disappearing until they applied their new method to the control of 
the yard and found that the amounts of tannin used and those found 
in the finished leather then checked easily within the limits of 
experimental error. 

In the manufacture of sole leather, one would expect the effects 
of aging to be much more pronounced. It seems reasonable to suppose 
that the Wilson-Kern method could be applied to a sole leather yard 
by keeping the tanned powders in the dried state for a sufficient length 
of time before washing to correspond to the conditions under which 
the sole leather was kept. That the A.L.C.A. method is no more 
reliable for heavy leather work than for upper leather is indicated by 
the following figures which have been made available to the author: 
Of 100 Ibs. of tannin, as determined by the A.L.C.A. method, that 
enter the leach house, only 39 lbs. appear as combined tannin in the 
finished leather. Losses in the 
spent tanning material, waste 
liquors, and water soluble matter 
from the leather were determined 
only by the A.L.C.A. method, but 
even with all this taken into con- 
sideration, there remains a large 
wilsed=kurn Wsthod loss that can be accounted for 
only on the assumption that the 
A.L.C.A. method gives _ results 
much too high. 


A.L.C.A, Method 


Apparent Percent. of Tannin 


Effect of pH Value. 


Thompson, Seshachalam, and 
Hassan** made a_ preliminary 
study of the effect of adding acetic 
and hydrochloric acids to extracts 
of quebracho, mimosa, mangrove, 

gambier, myrobalans, chestnut 
Fic. o1.—Effect of pH value upon Yooq and oak wood and found 
the determination of tannin in a : cE 

samplé-of quebracho extract, that the addition of small amounts 

. of acid affected practically all of 
the determinations made. Fig. 91, taken from a paper by Wilson and 
Kern,”> shows how the determination of tannin, by both the A.L.C.A. 
and the Wilson-Kern methods, is affected by change of pH value. The 
latter method gives a practically constant value over the wide range 3.6 
to 7.3. Where the falling off in per cent of tannin occurs at pH values 
higher than 7, indicated by the broken line, the results should not be con- 
sidered as found by this method because in each case the residual solu- 
tion gave a test for tannin, by the gelatin-salt test, whereas the method 
specifies that the determination is to be discarded whenever such a test is 


*4 Influence of Degree of Acidity on the Tannin Content of Solutions. F. C. Thompson, 
K. Seshachalam, and K. Hassan. J. Soc. Leather Trades Chem. 5 (1921), 380. 

°° Effect of Hydrogen-Ion Concentration upon the Analysis of Vegetable Tanning Ma- 
terials. J. A. Wilson and E. J. Kern. J. Ind. Eng. Chem. 14 (1922), 1128. 


2 4 6 8 10 
pH Value of Tan Liquor 


THE TANNINS 23 


obtained. The values obtained were included in the curve in order to 
show the effect of pH valtte on the rate of tanning. 


7 


Modified Wilson-Kern Method. 


Im order to meet the demand for a simpler method, Wilson and 
Kern 2° modified their method as follows: Standard hide powder is 
further purified by washing with water to free it from soluble matter, 
then dehydrating with alcohol, then soaking in two changes of xylene, 
and then drying. The tan liquor is filtered, as in the A.L.C.A. 
method, and only the soluble portion used, 100 cubic centimeters being 
shaken with 2 grams of purified hide powder for 6 hours. The 
tanned powder is allowed to wash over night in a specially designed 
percolator and is then dried and weighed. The increase in weight 
of the dry powder represents the weight of tannin in the roo cubic 
centimeters of solution used. Wilson and Kern compared the modified 
and original procedures of their method and found that they give 
practically identical results for all ordinary extracts. For further 
details, the original papers should be consulted. 


Potential Difference of Tannin Solutions. 


In Chapter 5 it was pointed out that the stability of a colloidal 
dispersion is determined less by the absolute value of the electrical 
charge on the particles than by the electrical difference of potential 
between the film of solution wetting the particles and the bulk of the 
surrounding solution. In the Procter-Wilson theory of tanning, to 
be discussed in Chapter 13, the astringency of a tan liquor in prac- 
tice is assumed to be a function of the potential difference between 
the solution immediately in contact with the tannin particles and the 
bulk of the tan liquor as well as of the potential difference between 
the tan liquor and the collagen jelly. Grasser *’ studied the electro- 
chemistry of tannin solutions, but obtained confusing results of rather 
doubtful value, which may be due to his failure to control or measure 
the hydrogen-ion concentrations of the liquors. 

Thomas and Foster *° were more successful. Using the U-tube elec- 
trophoresis method described by Burton,” they succeeded in measur- 
ing the potential differences of tannin solutions under different con- 
ditions. Table XXVIII shows a series of values obtained for tan 
liquors made from 8 typical tanning materials. It is interesting | 
to find gambier, the mildest tanning material, with the lowest potential 
difference and quebracho, the most astringent, with the highest poten- 
tial difference. The order of decreasing conductivity of these solutions 


26"Tne Determination of Tannin. J. A. Wilson and E. J. Kern. J. Ind. Eng. Chem, 13 
(1921), 772. 5 : : 

27 Electrochemistry of Tannins. G. Grasser. Collegium (1920), 17, 49, 277, 332. t 

28 The Colloid Content of Vegetable Tanning Extracts. A. W. Thomas and S. B. Foster. 
J. Ind. Eng? Chem, 14 (1922), 191. 

28 Physical Properties of Colloidal Solutions. E. F. Burton. Longmans, Green & Co., 
London (1916). 


232 THE CHEMISTRY OF LEATHER MANUFACTURE 


was sumac, gambier, oak bark, larch bark, hemlock bark, chestnut 
wood, osage orange, quebracho. It is evident that the potential differ- 
ence is not a simple function of the conductivity, but is influenced 
by the kind as well as the amount of electrolyte present. 


TABLE XXVIII. 


PoTENTIAL DIFFERENCES OF TANNINS FROM DIFFERENT SOURCES. 


Grams Potential 
Total Soluble Difference 
Extract Matter per liter volts 

Gambler s(Cubeyts ccs. wash ee se oe ee 18.7 — 0.005 
OakeDatiasce cere calcaneus ee 17.0 — 0.009 
Ciresiniutewood-thrsh ies ono 17.8 — 0.009 
Hemlockmoat Kit, o. tek cee 16.7 — 0.010 
SUMACT. + ictal buss «lia pee see 19.6 — 0.014 
BALCH SHAT Rawat sa he ak ale ae 19.5 — 0.018 
(Jsape ornige 7 tet ae ee 13:7 — 0.018 (?) 
Onebracho7= 2. oo vee ee eee 11.0 — 0.028 


If the absolute value of the electrical charge on the particles re- 
mains constant, according to the theory given in Chapter 5, the potential 
difference at the surface should decrease with increasing concentra- 
tion of electrolyte, or increase with decreasing concentration. Thomas 
and Foster found that the potential difference of solutions of quebracho 
extract actually does increase with decreasing concentration, as shown 
in Table XXIX. The addition of acid decreases the value of the 
potential difference by lowering the absolute value of the electrical 
charge, which holds true for negatively charged dispersions in general. 
This is shown in Table XXX. 


TABLE? ix 
PoTENTIAL DIFFERENCES OF SOLUTIONS OF QuEBRACHO EXTRACT, 
Concentration Potential 
Grams Dry Solids Difference 
per liter volts 

32 — 0.024 

16 — 0.028 

8 — 0.029 

4 — 0.030 
TABLE XXX. 


EFFECT OF ADDITION OF ACID. 


(16 grams of solid quebracho extract per liter.) 


0.1 N HCl added per liter Potential Difference 
cubic centimeters volts 
9) — 0.024 
10 — 0.014 
15 — 0.010 
20 approx. 0 


The effect of dialyzing a tan liquor is to lower the concentration 
of electrolyte, which we should expect to increase the potential dif- 


THE TANNINS 233 


ference. The values in Table XXXI show that this actually occurs, 
although part of the increase may be attributed to dilution. 


TABLE XXAXT 


EFFECT OF DIALYSIS. 


Potential 
Grams Extract Hours Final Difference 
Extract in 250 cc. Dialyzed Volume cc. volts 

OE a Ge 4 60 415 — 0.033 
PUSAUEUOEATIO“ iy oic.accescceces 4 24 370 — 0.024 
Oo ee 4 24 460 — 0.026 
Oa SS a ll 8.2 24 390 — 0.029 
BIeGMOCIeDaTK ....2........ oa 24 ove — 0.024 


Isoelectric Points of the Tannins. 


Thomas and Foster 2° later extended their investigations in an at- 
tempt to determine the isoelectric points of tannins from different 
sources. The various tanning extracts were dissolved in a citrate 
buffer mixture having a pH value of 2.0 and the solutions were finally 
adjusted to the desired pH values by means of the hydrogen electrode. 
The buffer was apparently necessary to eliminate, or delay, the sec- 
ondary actions, such as diffusion of the boundaries and change of re- 
action of the extracts due to electrolysis, which behavior had nullified 
previous experiments. 

Between the pH values 2.5 and 2.0, the direction of migration of 
the tannin particles changed from anodic to cathodic in solutions of 
the extracts of oak bark, hemlock bark, wattle bark, sumac, and 
gambier. In the case of quebracho, there seemed to be no movement 
in the U-tube at the pH values 3.0 or 2.5, but at 2.0 the movement 
seemed to be slightly cathodic. Quebracho was precipitated by the 
buffer and only the clear, supernatant liquor could be used, which may 
account for the inability to obtain more definite results. 

Until they are located more definitely, the isoelectric points of 
the tannins may be accepted as lying between the pH values 2.0 and 2.5, 
at least those of hemlock, oak, and wattle barks, sumac, and gambier. 


Precipitation of Tan Liquors. 


In the hope of throwing some light upon the colloidal nature of the 
tannins, Thomas and Foster studied the action of various electrolytes 
upon a great variety of tan liquors. Aqueous solutions of different 
tanning extracts were made up so that 100 cubic centimeters of solu- 
tion contained 4 grams of solid matter. The solutions were made at 
85° C., cooled to 25°, and then adjusted to final volume. The stock 
solution was then centrifuged for 5 minutes at 1000 times gravity in 


80 The Electrical Charge of Vegetable Tannin Particles. A. W. Thomas and S. B. 
Foster. Ind. Eng. Chem. (1923); (advance copy). 


234 THE CHEMISTRY OF LEATHER MANUFACTURE 


order to throw down coarse suspended matter. Portions of 25 cubic 
centimeters were put into 100-cubic centimeter, graduated oil tubes. 
Then 25 cubic centimeters of the electrolyte were added, the solutions 
were allowed to stand for 15 to 30 minutes for precipitation to start, 
and were then centrifuged for 5 minutes at 1000 times gravity, The 
volumes of the precipitates were recorded and plotted against the 
concentrations of electrolyte employed. 

The results may be most conveniently studied by grouping them 
wader the names of the various electrolytes used. Each available ex- 
tract was not tested with all electrolytes because, in some cases, pre- 
liminary experiments indicated that further work would be fruitless. 


Monovalent Cations. 


Potassium chloride. Concentrations of potassium chloride from 
0.02 to 4 molar gave only negligible amounts of precipitate with gambier 
and quebracho. Oak bark gave a gradually increasing salting out effect. 


Oak Bark 
8) Larch Bark 
Hemlock Bark | 
Chestnut Oak Bark 


4 Quebracho 


Vol. of Precipitate (c.c.) 


Vol. Precipitate (c.c.) 


MM MM M M M 234 6M MM MM MM M oM 


10050 2510 4 2 7 10050 2010 495% 
Concentration of Acid Concentration of Sulfuric Acid 
Fic. 92.—Precipitation of Tannins by Hie 03.—Precipitation of Tannins by 
Hydrochloric and Phosphoric Acids. Sulfuric Acid. © 


Since gambier and quebracho represent extreme types of tanning ex- 
tracts, no further tests were made with this salt. It must be borne 
in mind that the solutions to which the neutral salts were added were 
made simply by dissolving the extracts in distilled water and had pH 
values in the vicinity of 4.5. 

Hydrochloric acid. Concentrations from 0.01 to 6 molar were 
used. Gambier and quebracho gave large amounts of precipitate only 
at high concentrations of acid and, since this was not a simple colloid 
precipitation, no further experiments were attempted. A salting out 
effect was obtained with oak bark. (See Fig. 92.) 

Sulfuric acid. Quebracho, hemlock bark, oak bark, and larch bark 
gave progressively increasing amounts of precipitate with increasing 
concentration of acid, as shown in Fig. 93. No precipitate was ob- 
tained with sumac until molar concentration was reached, when gummy 


THE TANNINS 238 


masses were thrown down, similar to those obtained with aluminum 
sulfate. At 4 molar concentration, a flocculent precipitate was formed. 

Phosphoric acid. Gambier began to give an appreciable precipitate 
only at 4 to 7 molar concentration. With sumac a gummy mass was 
thrown out at 2 molar, as was observed upon the addition of sulfuric 
acid and aluminum sulfate, and at 4 to 7 molar a flocculent precipi- 
tate formed which left the supernatant solution almost colorless. 
Quebracho was progressively salted out. (See Fig. 92.) 

Acetic acid. Experiments with quebracho, sumac, gambier, and 
oak bark were run with concentrations of acid from 0.005 to 4 molar. 
There was no appreciable precipitation in any case. At the higher 
concentrations the suspended matter began to dissolve. 

Formic acid. Concentrations from 0.005 to 12.5 molar were used. 
Sumac, chestnut oak bark, larch bark, gambier, and hemlock bark 


oi 


» 


tae) 


ke 
(lactic) 4 
ae uw UM UM UM C2 34M 
6077 .20)-10 Mi an 


“ uebracho 
(formic) » 
Vol. of Precipitate (c.c.) 
«a 


Vol, Precipitate (c.c.) 


ow eo ee 
300 100 BO 25 10 6 @ 


Concentration of Acid Concentration of Barium Chloride 
Fic. 94.—Precipitation of Tannins by Fic. 95.—Precipitation of Tannins 
Formic and Lactic Acids. by Barium Chloride. 


gave no precipitation up to 4 molar, at which concentration the sus- 
pended matter began to dissolve. Quebracho and quercitron bark were 
precipitated, but the precipitate redissolved at from 2 to 4 molar. 

(See Fig. 94.) 

. Lactic acid. Concentrations from 0.005 to 2 molar were employed. 
The effects of this acid were similar in kind, but not in degree, to 
those with formic acid. (See Fig. 94.) The precipitates with que- 
bracho and quercitron redissolved at lower concentrations of lactic 
than of formic acid. Since lactic is the weaker acid and since this 
redissolving was not found with hydrochloric or sulfuric acids, the 
effect must be due to chemical properties other than those of the 
hydrogen ion. This is an important point to consider in the chemical 
control of tan liquors. 


Divalent Cations. 


Barium chloride. On account of the limit of solubility, this salt 
was employed up to only 0.5 molar. The salting out effect is shown 


in Fig. 95. 


236 THE CHEMISTRY OF LEATHER MANUFACTURE 


Calcium chloride. Concentrations up to 2 molar were used. As 
with barium chloride, increasing amounts of precipitate were obtained 
with the different tanning materials used, as shown in Fig. 96. At the 
same concentration of these salts the different extracts gave in some 


Fw 8 FF HN HD 3 


oo 
? “e, 


/ Gambier ™--..\ 


MM M MM MM M 


Volume of Precipitate (c.c,) 


Vol. of Precipitate (c.c,) 


MMMM MM M M 2 3M 


20010050 2010 4 B T 800 40020010060 2010 4 
Concentration of Calcium Chloride Concentration of Aluminum Sulfate 
Fic. 96.—Precipitation of Tannins F'1c. 97.—Precipitation of Tannins by 

by Calcium Chloride. Aluminum Sulfate. 


cases less, and in others more, precipitate, showing the presence of 
substances reacting with barium and calcium ions to form compounds 
of different solubilities. 


Trivalent Cation. 


Aluminum sulfate. In the pre- 
cipitation of negatively charged 
colloidal dispersions, aluminum 
sulfate is not only a powerful pre- 
cipitant, but it also gives the “ir- 
regular series” or “tolerance zone” 
which is typical of the action of 
weak base cation-strong acid anion 
salts, as shown by Buxton and 
Teague,** and by Freundlich and 
Schucht.*? The concentrations of 
aluminum sulfate used ranged 
from 0.00125 to 0.5 molar. 

The “irregular series” effect 
was obtained with gambier, sumac, oak bark, and quercitron bark. 
Precipitation generally set in at 0.00125 molar concentration, rose 
rapidly to a maximum, dropped off into a “tolerance zone,” and then 
started upward again, as shown in Fig. 97. 

Those which gave no “irregular series,” at least up to 0.5 molar 
concentration of the salt, were osage orange, quebracho, camel cutch, 
chestnut wood, chestnut oak bark, hemlock bark, and larch bark, shown 


%Z. physik. Chem. §7 (1907), 76. 
82 ITbid., 85 (1913), 641. 


Vol, Precipitate (c.c,.) 
F wo YQ fF TD DH 
\“Hemlock Bark 


& 
2 
3) 
= 
Gg 
M 


MMM MM OM OM 
800400200100 50 20 10 4 
Concentration of Aluminum Sulfate 


Fic. 98.—Precipitation of Tannins by 
Aluminum Sulfate. 


THE TANNINS 237 


Hydrochloric Acid 


Sulfuric Acid 
Formic Acid 


Volume of Precipitate (c.c.) 


ai 2 3 4 5 6 7 
pH Value of Tan Liquor 
Fic. 99.—Precipitation of Tannins of Quebracho Extract as a Function 
of pH Value. 
in Fig. 98. Precipitation started at 0.00125 molar and increased grad- 
ually to about 0.1 molar, where there was an abrupt upward trend 
similar to a salting out effect. These extracts are not so sensitive to 
precipitation by dilute solutions of aluminum sulfate as those shown 


Hydrochloric Acid 
—o—_—_o—__—__6— 


Sulfuric Acid 
———_@—__—___@__"__ 


Volume of Precipitave (c.c.) 


af 2 3 4 5 6 7 


pH Value of Tan Liquor 


Fic. 100.—Precipitation of Tannins of Gambier Extract as a Function of 
pH Value. 


238 JHE CHEMISTRY ON LEATHER MANUFACTURE 


in Fig. 97. Bengal cutch seemed to be in a separate category, since 
It was unaffected by the addition of aluminum sulfate. 


Hydrogen-Ion Concentration. 


The effect of hydrogen-ion concentration upon the precipitation of 
solutions of quebracho, gambier, larch bark, and oak bark by sulfuric, 
hydrochloric, and formic acids is shown in Figs. 99, 100, IoI, and 
to2. Solutions of sumac, hemlock bark, and wattle bark were not 
precipitated by these acids with increasing acidity to pH =1. It is 
evident that the volume of precipitate formed is not a function of 


Hydrochloric Acid 
—O-—_—_—_- ee 


w 
es 
\) 


Sulfuric Acid 
—___9____ —_q—___ 


n 
e 
oi 


Formic Acid 


Volume of Precipitate (c.c.) 
hw = bo 
ro) roy ro) 


Oo 
e 
oO 


pH Value of Tan Liquor 


Fic. 101.—Precipitation of Tannins of Larch Bark Extract as a Function 
of pH Value. 


hydrogen-ion concentration alone, since the three acids give curves of 
different shapes. 

Wherever a precipitate formed, the amount invariably increased 
with increasing hydrogen-ion concentration where hydrochloric and sul- 
furic acids were used. But an increasing concentration of formic acid 
dissolved the precipitate, or the suspended matter in cases where no 
precipitate had previously formed. 

The precipitates obtained with hydrochloric acid were found to 
be soluble in strong alcohol and in 9 molar lactic acid. On shaking 
up with water, these precipitates dispersed, but gradually settled out 
more or less completely in 24 hours. In the case of oak bark and 
quebracho, it was found that approximately two-thirds of the original 
solid matter present had been precipitated at pH = 1. 


THE TANNINS | 239 


When the pH value was increased by the addition of sodium 
hydroxide, there was increasing solution, clear liquids being obtained in 
every case at pH = 8. The effect of adding calcium hydroxide, how- 
ever, is very different, as will be recalled from Fig. 87 of Chapter 11. 


Hydrochloric Acid 
—o—__9————_6— 


Sulfurie Acid 
Formic Acid 


Volume of Precipitate (c.c.) 


1 Z 3 4 s 6 7 
pH Value of Tan Liquor 


Fic. 102.—Precipitation of Tannins of Oak Bark Extract as a Function of 
pH Value. 


At pH values above 7.2 increasing amounts of precipitate are obtained 
with increasing pH value. 

The conduct of the extracts examined by Thomas and Foster shows 
that they contain.a large amount of colloidal matter of a type of disper- 
sion with properties between those of the intermediate and hydrophilic 
dispersions. From the colloidal point of view, vegetable tanning ma- 
terials furnish an almost unexplored field; the work outlined in this 
chapter cannot be considered as more than a good start. 


Chapter 13. 
Vegetable Tanning. 


Raw skin is readily putrescible in the wet state. Upon drying, the 
collagen fibers become glued together and the skin becomes very stiff. 
Although the dried skin will not putrefy, it again becomes putrescible 
as soon as it comes into contact with water. Thousands of years 
ago the discovery was made that the properties of skin substance change 
completely when the wet skin is brought into contact with the aqueous 
extract of those forms of plant life which have since come to be 
classed as vegetable tanning materials. The action which brings about 
this change of properties is known as vegetable tanning and the com- 
pound of skin protein and tannin as leather. Under normal con- 
ditions, the fibers of leather do not glue together upon drying and they 
are not putrescible even in the wet state. 

The practice of tanning is greatly complicated by the necessity 
for endowing the leather with many delicate properties, according 
to the use to which it is to be put, all of which are markedly affected 
by slight differences in manipulation. The effect produced by any 
single change in the tanning process depends upon the nature of every 
one of the numerous operations preceding and following that in which 
the change has been made. In the manufacture of one type of leather, 
a skin may be subjected to scores of different operations and a slight 
change in any one of these may necessitate changes in nearly all of the 
others in order to preserve the specific properties desired in the finished 
leather. It is this fact that renders most practical treatises of leather 
manufacture of so little value to the tanner. Were he to try to adopt 
an operation described in the literature which was better in itself than 
the one he was using, he might find that the change would spoil his 
leather because of its failure to harmonize with all of the other opera- 
tions peculiar to his particular process. There are, however, certain 
broad principles of tanning which are followed generally. 

‘Two conditions may be accepted as essential to successful tanning: 
the first that the natural physical structure of the skin shall be changed 
but very little; the second that the degree of tannage shall be as nearly 
uniform as possible throughout the skin. The second condition, in a 
large measure, is essential to the first. 

The physical means widely adopted to preserve the natural struc-_ 
ture of the skins during tanning is to suspend them freely from sticks 
with the heads hanging downward in the tan liquors, care being taken 
to see that the unhaired skin is free from creases or wrinkles, which 

240 


VEGETABLE TANNING 241 


would be permanently fixed by the tannage. Usually the lower end 
of each skin is tacked onto a stick and the skin is then spread out 
carefully so that it hangs in its natural condition when immersed in 
the tan liquor. The supporting stick rests upon a rectangular frame 
floating in the liquor. The skins are not subjected to any violent 
mechanical agitation until the grain surface has been “set” by the 
tannage and the tannins have penetrated into the skin for a considerable 
distance. 

If skins from the beamhouse were put directly into strong tan 
liquors of such reaction that the rate of combination of tannin with 
the skin protein was abnormally great compared to the rate of dif- 
- fusion of tannin into the interior of the skin, the tendency for the 
outer layers to assume an area different from that of the skin as a 
whole would cause a distortion of the skin that would be permanent. 
In such a case, the liquor is called very astringent. The fact is often 
overlooked that the reaction of the solution previously absorbed by 
the skin may be as important in bringing about this condition as the 
reaction of the tan liquor itself. In fact a given tan liquor may appear 
very astringent to a pickled skin and yet very mild to a skin taken 
directly from the bate liquor. The distortion may show itself as coarse 
wrinkles, as the finer reticulation illustrated in Fig. 48 of Chapter 5, 
or merely as a rough grain surface. Since these distortions greatly 
lower the value of the leather, every effort is made to avoid them. 

The practical means adopted by the tanners to eliminate this danger 
is to hang the skins from the beamhouse first in a tan liquor which 
has been used to tan a great many lots of skins previously and in which 
the ratio of nontannin to tannin is very great. Each day the skins 
are then moved into stronger and fresher liquors until completely 
tanned. The effect of an increasing ratio of nontannin to tannin in 
the tan liquor is to increase the ratio of the rate of diffusion of the 
tannin into the skin to the rate of combination of the tannin with 
the skin protein. This has the obvious effect of making the rate of 
combination more uniform throughout the skin, and consequently 
lessening the tendency towards distortion. The ideal process would 
be the one in which combination was entirely prevented until the tannin 
was uniformly distributed throughout the skin and then allowed to 
proceed uniformly by a suitable change of reaction of the liquor. An- 
other safeguard which tanners have been forced to adopt, without 
understanding its mechanism, is so to regulate the reactions of both 
the tan liquors and the solution absorbed by the skin proteins just 
prior to tanning that the tanned and untanned portions of the skin 
protein do not tend to assume greatly different specific volumes. 

The progress of the diffusion of the tan liquor into the skin is 
determined by cutting off a strip and observing the color of the freshly 
exposed portfon. The raw portion is white and the tanned layers 
usually a deep brown. When the tannin has penetrated almost to 
the middle of the skin, it is customary to take the skins off from the 
sticks and pile them into vats known as handlers or layers. The name 
handler is used when the skins are handled from vat to vat at fre- 


242 THE CHEMISTRY OF LEATHER MANUFACTURE 


quent intervals until completely tanned. The name layer is used 
for the vats in which heavy hides are laid away for long periods, 
during which the tannin diffuses very slowly into the interior. Although 
hardly more than a week is consumed in the diffusion of the tan liquor 
into a light skin, months are required in some processes of sole leather 
manufacture. : 

Where great solidity is required, as in sole leather, it is not sufficient 
merely to convert all of the collagen into leather. The volume of the 
collagen fibers increases as more tannin combines with them. After 
the hides have become completely colored throughout, it is customary 
to treat them with very strong tan liquors with the object of getting 
as much tannin fixed as possible, and mechanical agitation of one 
kind or another is often employed. Usually the weight of sole leather 
is further increased by the incorporation of glucose and magnesium 
sulfate in the leather. 


The Structures of Tanned Skins. 


In the manufacture of leather for definite purposes, the choice of 
the kind of skin is of the greatest importance. By varying the nature 
of the tanning process, the properties of the leather can be varied, 
but not sufficiently to make one kind of skin suit all purposes. Advan- 
tage is taken of the variety of skins furnished by nature in order to 
simplify the tanning process itself. 

Fig. 103 shows a vertical section taken from the butt of a steer 
hide tanned for sole leather. The natural solidity of this hide 1s so 
great that a heavy degree of tannage would not have been necessary 
in order to produce a leather suitable for shoe soles. This particular 
leather was heavily tanned with oak bark extract, but was not loaded — 
with glucose and magnesium sulfate. 

A section of vegetable tanned calf skin is shown in Fig. 143 of 
Chapter 14, where it was put for direct comparison with chrome tanned 
calf made from part of the same skin. It is interesting to compare 
its structure after tanning with that of calf skin in the fresh state, 
shown in Fig. 18 of Chapter 2. The leather is typical of the finest 
grade of finished shoe upper leather. 

Fig. 104 shows a section of vegetable tanned sheep skin just as 
it came from the tan liquors. Note the great contrast which it pre- 
sents to the leather made from steer hide or calf skin. The holes and 
empty spaces left by the wool and glands give the leather a spongi- 
ness that makes it unsuitable for many purposes. The upper layer 
is often split from the rest of the skin and used in bookbinding, for 
hat bands and for the linings of expensive shoes instead of cloth. 
Sheep skin leather is sometimes used as a substitute for kid leather in 
the manufacture of gloves, where its softness is an asstt. The raw 
skin is shown in Fig. 28. | 

Fig. 105 is a section of vegetable tanned leather from the butt, 
or shell, of a horse hide. This is finished leather ready for use in 
the manufacture of the uppers of heavy, waterproof shoes. The raw 


VEGETABLE TANNING 243 


hide, at much lower magnification, is shown in Fig. 31. It will be 
noted that the leather has been split into layers through the portion 
known as the glassy layer, only the upper layer being used. The 
compactness of the fibers in the bottom third of the leather makes 
it waterproof and almost airtight. Leather from this part of the horse 
hide is known as Cordovan. The section should be compared with 
Fig. 145 of Chapter 14, which shows a section from the same butt 
which has been chrome tanned; the contrast is. striking. 

The peculiarity of the horse hide is that this compact fibrous 
structure is found only in the butt, the rest of the hide being very 
loose in texture. Fig. 106 shows a section taken from the same piece 
of leather as that shown in Fig. 105, but from a point further up 
the back beyond the boundary of the glassy layer. Its softness and 


-. sponginess has found for it a use in the manufacture of heavy 


gloves. 
The section shown in Fig. 107 is that of a vegetable tanned hog skin. 


A section of the fresh skin is shown in Fig. 30. When the flesh 
side of the leather was shaved to make it smooth, the bottom of 
the pocket of the hair follicle was cut away, leaving the hole running 
right through the leather, as shown in the figure. This is typical 
of hog leathers; wherever there were bristles in the original skin, 
holes pierce the final leather. The roughness of the grain surface 
of the leather gives it a place in the manufacture of saddles, football 
covers, purses, etc. | ) 

Fig. 108 shows a vertical section of salmon leather taken directly 
from the vegetable tan liquors. It should be compared with the 
section of fresh skin shown in Fig. 36. The gap in the upper portion 
is the follicle once occupied by a scale. The structure of the leather 
makes it suitable for belt lacings. 

The raspy feel of certain kinds of shark leather is explained by 
the section shown in Fig. 109. Shark leather has recently been tried 
for shoe uppers, in which case the hooks are removed prior to tanning. 
The fibrous structure resembles that of other fishes. 

Fig. 110 shows a vertical section of vegetable tanned alligator skin. 
This type of leather finds an outlet in the manufacture of bags and 
cases. Fig. 111 shows a section of leather made from the skin of a 
horned toad.. Although these skins are very small, they make very 
pretty doilies and fancy purses. In both the alligator and toad skins, 
the fibrous structure resembles that of the fishes. 

A section of leather made from camel skin is shown in Fig. 112. 
It is remarkable for its compact structure, which would make it suit- 
able for belting leather or for light soles. Figs. 113 and 114 show 
portions of the section of a vegetable tanned walrus hide and Figs. 
Ir5 and 116 show sections of the tanned hide of a hippopotamus.* 
The most remarkable thing about these hides is their great size. The 
actual thickness of the walrus leather was 24 millimeters and that of 
the hippopotamus leather 30 millimeters. In order to show the entire 


1The hippopotamus, walrus, and camel leathers were very kindly furnished by Professor 
Douglas McCandlish of the University of Leeds, England, , : oLessol 


ra 


v 4 Co 
Ge gee ge ge} 
cae, ae “e 


Pe i) 


Fig. 103.—Vertical Section of Steer Hide Leather. 
(Sole leather.) 


Eyepiece: none. 

Objective: 48-mm. 

Wratten filter: H-blue green. 
Magnification: 15 diameters, 


Location: butt. 

Thickness of section: 40 pw. 
Stain: none. 

Tannage: vegetable. 


244 


Fig. 104.—Vertical Section of Unfinished Sheep Leather. 


Location: butt. 


Thickness of section: 30 u. 
Stain: none. 


Tannage: vegetable. 


245 


Eyepiece: none. 

Objective: 16-mm. 

Wratten filter: H-blue green. 
Magnification: 46 diameters. 


Fig. 105.—Vertical Section of Horse Leather. 
(Cordovan—from shell.) 


Location: butt. Eyepiece: none. 

Thickness of section: 20 u. Objective: 16-mm. 

Stain: Daub’s bismarck brown. Wratten filter: H-blue green. 
Tannage: vegetable. Magnification: 70-diameters. 


246 


Fig. 106.—Vertical Section 
(From spongy part of back near shell. ) 


Thickness of section: 20 u. Objective: 16-mm. 
Stain: Daub’s bismarck brown. Wratten filter: H-blue green. 
Tannage: vegetable. Magnification: 70 diameters. 


of Horse Leather. 
Location: back. 


Eyepiece: none. 


247 


Fig. 107.—Vertical 


Location: butt. 


Thickness of section: 30 u. 
Stain: none. 


Tannage: vegetable. 


248 


Section of Hog Leather. 


Eyepiece: none. 

Objective: 16-mm. 

Wratten filter: K3-yellow. 
Magnification: 46 diameters. 


Fig. 108.—Vertical Section of Unfinished Salmon Leather. 


Location: side. 

Thickness of section: 20 u. 
Stain: none. 

Tannage: vegetable. 


Eyepiece: 5X. 

Objective: 16-mm. 

Wratten filter: B-green. 
Magnification: 110 diameters. 


249 


Fig. 109.—Vertical Section of Shark Leather. 
Location: (?). 


Thickness of section: 50 uw. 
Stain: none. 


Tannage: vegetable. 


Eyepiece: 5X. 

Objective: 16-mm. 

Wratten filter: B-green. 
Magnification: 75 diameters. 


250 


Fig. 110.—Vertical Section of Alligator Leather. 
Location: back. 


Thickness of section: 50 uw. 
Stain: none. 


Tannage: vegetable. 


Eyepiece: none. 

Objective: 16-mm. 
Wratten filter: G-yellow. 
Magnification: 45 diameters. 


251 


Fig. 111.—Vertical Section 


Location: butt. 

Thickness of section: 20 u. 
Stain: none. 

Tannage: vegetable. 


252 


of Horned-Toad Leather. 


Eyepiece: 5X. 

Objective: 8-mm. 

Wratten filter: H-blue green 
Magnification: 220 diameters. 


Fig. 112.—Vertical Section of Camel Leather. 


Location: butt(?). Eyepiece: none. 

Thickness of section: 30 wu. Objective: 32-mm. 

Stain: none. Wratten filter: B-green. 
Tannage: vegetable. Magnification: 30 diameters. 


293 


Portions of Vertical Section of Walrus Leather. 
Fic. 113.—Region of Grain Surface. 
fic, 114.—Region 22 Millimeters Below Grain Surface. 


Location: butt(?). Eyepiece: 5X. 
Thickness of section: 40 p. Objective: 16-mm. 


Stain: none. Wratten filter: G-yellow. 
Tannage: vegetable. Magnification: 68 diameters, 


254 


Portions of Vertical Section of Hippopotamus Leather. 
Fic. 115.—Region of Grain Surface. 
Fic. 116.—Region 28 Millimeters Below Grain Surface. 


Location: butt(?). Eyepiece: 5X. 

Thickness of section: 40 wn. Objective: 16-mm. 

Stain: none. Wratten filter: G-yellow. 
Tannage : vegetable. Magnification: 68 diameters, 


255 


ie 


256 THE CHEMISTRY OF LEATHER MANUFACTURE 


thickness of the leather at 68 diameters, a picture about seven feet 
high would be required. 

The walrus must be very sensitive to touch, if we may judge 
from the highly developed papillae which protrude everywhere from 
the grain surface. In neither of these leathers were the roots of the 
hair removed and the fat cells surrounding the hair bulbs of the 
walrus were still-intact as though the unhairing liquors had not pene- 
trated that deeply. Except for the huge collagen fibers in the reticular 
layer, and the great size of the hide, the walrus hide resembles that 
of the common hog. It is interesting to compare the fibers of these 
leathers with those of the smaller skins, but the differences in 
magnification must be taken into consideration. 

It has often been supposed that the tanning action consists of a 
coating of the skin fibers with tannin, but observations of sections 
under the microscope indicate that this is not the case. The outer 
surfaces of the skin act as filters, permitting only the soluble matter 
to pass into the interior, where it subsequently diffuses into the sub- 
stance of the fibers, which assume a uniform color throughout when 
tanning is finally complete. In finished leather, contrary to what seems 
to be the general belief, we find no coating of the surfaces of the fibers 
nor any material precipitated in the spaces between them. 


Rate of Diffusion of Tan Liquor into Gelatin Jelly. 


The great length of time required to tan heavy leathers is due to 
the very slow rate of. diffusion of the tannin into the interior of the 
hides. Because of the difficulty of measuring the extent of pene- 
tration of tan liquors into raw hides, studies of the rate of diffusion 
are usually made with tubes of gelatin jelly. Hoppenstedt ? noted 
that different tanning extracts diffused into gelatin jelly at different 
rates, the order of increasing rate of diffusion being mangrove bark, 
quebracho, hemlock bark, algarobilla, valonia, oak bark, myrobalans, 
chestnut wood, gambier, divi-divi, sumac. 

Later Thomas ** showed that the rate of diffusion of tanning ex- 
tracts into gelatin jelly increases with the ratio of nontannin to tannin 
in the extract. For typical samples, he found the rate of diffusion 
increasing in the order quebracho, hemlock bark, larch bark, oak bark, 
chestnut wood, gambier, sumac, agreeing with the results obtained 
by Hoppenstedt. This is also the order for decreasing astringency of 
these materials, as ordinarily used. The same order is roughly borne 
out in experiments dealing with the rate of diffusion into cow hide. 

The action of nontannins in increasing the rate of diffusion of 
tannins into skin may be explained as follows: Tannins and cer- 
tain nontannins form compounds with collagen, but the collagen-tannin 
compound is very stable, while the collagen-nontannin compounds are 

* Diffusion of Tannins through Gelatin Jelly. A. W. Hoppenstedt. J. Am. Leather 
Chem, Assoc. 6 (1911), 343. 


*a Order of Diffusion of Tanning Extracts through Gelatin Jelly. A, W. Thomas. J. Am. 
Leather Chem, Assoc. 15 (1920), 593. 


MEGEITABLE TANNING 257 


considerably dissociated. The nontannins, having a much smaller 
molecular weight than the tannins, diffuse more rapidly into the 
skin. When the slowly moving tannin reaches a point where it would 
combine with collagen, it cannot do so because the point is already 
occupied by nontannin. Tannin that would otherwise have combined 
with collagen near the surface of the skin is thus enabled to pro- 
ceed into the interior and the measured rate of penetration is thereby 
increased. ‘This action is more marked the greater the concentration 
of nontannin capable of combining with collagen. The collagen-tannin 
compound being much the more stable, tannin replaces nontannin as 
fast as the collagen-nontannin compound hydrolyzes. 

According to the Procter-Wilson theory of tanning, to be dis- 
cussed presently, the rate of tanning, and also of the combination of 
collagen with certain nontannins, can be decreased either by increasing 
the electrolyte concentration or by lowering the positive electrical 
charge which collagen possesses in acid solution, which can be accom- 
plished by decreasing the acidity. We should therefore expect the 
constituents of a tan liquor, both tannin and nontannin, to penetrate 
skin more rapidly as the acidity of the tan liquor is decreased to the 
isoelectric point of collagen. 

Thomas prepared a 5-per cent dispersion of gelatin in hot water 
containing 0.1 per cent ferric chloride and poured it into a series of 
test tubes to three-quarters of their capacity. When the dispersions 
had set to jelly, equal volumes of solutions of different extracts were 
poured on top of the jellies, which were then placed in an ice box. 
All of the extract solutions were made to contain 1 per cent of dry 
solid matter. Tannin and some nontannins react with ferric chloric 
giving very deep green or blue colors, which served to indicate the 
extent of the penetration. In 96 hours the gambier had penetrated 
18.0 millimeters as against only 4.8 millimeters by the quebracho. It 
was, of course, the extent of penetration by certain nontannins that 
was measured, as these diffuse more rapidly than the tannin. 

Wilson and Kern * treated a large volume of a dispersion of gelatin 
in dilute ferric chloride solution with tartaric acid until its pH value 
was reduced to 2.5, as determined by the hydrogen electrode. Equal 
portions were then treated with sodium hydroxide to give the desired 
pH values, which ranged from 2.5 to 11.0. Dilutions were such that 
the final dispersions contained 5 per cent of gelatin and 0.1 per cent 
of ferric chloride, as in the experiments of Thomas. 

Solutions of gambier and quebracho extracts were treated with 
tartaric acid to give a pH value of 2.5. Equal portions were then 
treated with sodium hydroxide to give series of pH values the same 
as in the series of jellies. Each final liquor contained 1 gram of solid 
matter of the original extract per 100 cubic centimeters. aeaye 

The gelatin dispersions were poured into test tubes and allowed 
to set. Onto each was poured a given volume of tan liquor having 
the same pH value as the jelly. Both the quebracho and gambier series 


* Effect of Change of Acidity upon the Rate of Diffusion of Tan Liquor into Gelati 
Jelly. J. A. Wilson and E. J. Kern. J. Ind. Eng. Chem. 14 (1922), 45, * a 


2588 THE CHEMISTRY OF LEATHER MANUFACTURE 


were run in duplicate. They were kept in the ice box and examined at 
intervals for 96 hours. The extent of the diffusion of the tan liquors 
into the jellies is shown in Fig. 117, the measurements being taken 
after 96 hours. In each case the duplicate series were practically 
identical. 

Gambier, which has a high 
ratio of nontannin to tannin, be- 
gins to penetrate at a pH value of 
3.0 and reaches its maximum rate 
at pH = 6.0. Quebracho, on the 
other hand, scarcely shows any 
penetration until pH = 4.7, the 
isoelectric point of gelatin, is 
reached. At pH values greater 
than 9, however, the quebracho 
liquor penetrates at the greater 
rate, possibly because of its higher 
tannin content. 

Studies were also made of the 
effect of change of pH value upon 
Sq hape ae the rate of diffusion of tan liquors — 

7 8 9 10 into cow hide. With increasing 

pH Value pH values up to about 8, there is 

Fic. 117.—Rate of Diffusion of Tan 4 distinct increase im rate Of dit- 

Liquor into Gelatin Jelly as a Func- fusion, but because of the flaccid 

tion of pH Value. nature of hide at pH = 8 it is dif- 

ficult to make accurate measure- 

ments of the rate of diffusion. At pH values below 3 and above 11 the 
hide swells considerably and becomes rubbery and distorted. 


Extent of Diffusion in Millimeters 


Rate of Tanning as a Function of Time and Concentration of 
Tan Liquor. 


An extremely important series of investigations of the nature of 
the vegetable tanning process has recently been begun by Thomas and 
Kelly, which promises to throw much light on the mechanism of this 
very complex process. Their first studies*® were devoted to the ef- 
fects of time and concentration. In their preliminary experiments, por- 
tions of purified hide powder were shaken with definite quantities of un- 
filtered solutions of tanning extracts for stated lengths of time, washed 
free from soluble matter, and then analyzed for the purpose of deter- 
mining the amount of tannin combined with a unit of hide substance. © 
In the more concentrated liquors, however, an error was introduced 
by the occlusion of insoluble matter by the hide powder, which was 
included as combined tannin because it was not removed later by 
washing. 


Time and Concentration Factors in the Combination of Tannin with Hide Substance. 
A. W. Thomas and M. W. Kelly. J. Ind. Eng, Chem. 14 (1922), 202. 

5 The Concentration Factor in the Fixation of Tannins by Hide Substance. Ibid. (1923); 
(advance copy). 


VEGETABLE TANNING 209 


In their most recent work, Thomas and Kelly adopted a method 
practically identical with the modified Wilson-Kern method of tannin 
analysis described in Chapter 12, except for the fact that no attempt 
was made to detannize the various solutions completely. All tan 
liquors were centrifuged and filtered and only the clear filtrates used 
in the experiments. The use of filtered liquors with hide powder gave 
results which were more uniform and which probably represent actual 
tanning conditions more. closely, since the surfaces of the whole skin 
act as filters, permitting only the soluble matter to come into contact 
with the great bulk of the skin protein. 


WATTLE BARK 
pH = 4.1 


Grams Tannin Fixed by 100 Grams Hide Substance 
Grams Tannin Fixed by 100 Grams Hide Substance 


20 40 60 80 100 120 140 20 40 60 80 100 120 140 


Grams Solid Matter per Liter ; Grams Solid Matter per Liter 

Fic. 118—Rate of Tanning as a Fic. 119.—Rate of Tanning as a 

Function of the Concentration of Function of the Concentration of 
Tan Liquor. Time, 24 hours. Tan Liquor. Time, 24 hours. 


Portions of purified hide powder equal to 2 grams of anhydrous 
substance were shaken with I00 cubic centimeters of tan liquor of 
the desired concentration and for fixed intervals of time. The powder 
was then washed until the. wash water no longer gave a dark color 
upon the addition of a drop of ferric chloride solution. It was found 
that the ferric chloride test is capable of detecting I part in 75,000 
of either gallic acid or pyrogallol. The powders, freed from soluble 
matter, were dried in a current of warm air and then completely 
dried in the oven. The increase in weight of the absolutely dry ma- 
terial was taken as the amount of tannin fixed by 2 grams of hide 
powder. 

Figs. 118 and 119 show how the rate of tanning varies with in- 
creasing concentration of solutions of quebracho, hemlock bark, larch 


260 THE CHEMISTRY OF LEATHER MANUFAC 


bark, gambier, oak bark, and wattle bark extracts. The mild action 
of gambier, as contrasted with the astringency of quebracho, is 
graphically shown by the steep rise of the quebracho curve compared 


Grams Tannin Combined with 100 Grams Hide Substance 


GAMBIER. GAMBIER 
(39 grams per liter) (76 grams per liter) 


24 hours . e4 hours 


AK BA 
(43 grams per liter) (88 grams per liter) 


24 hours 24 hive 


WATTLE BARK WATTLE BARK 
(37 grams per liter) (61 grams per liter) 


(Concentrations are given 


in terms of dry solid 
matter) 


2 weeks 


24 hours 


£1 4 6 8 ee ee 2 4 6 82 0a 
pH Value of Tan Liquor 


Fic. 120.—Rate of Tanning as a Function of pH Value. 


with that of the gambier series. It is remarkable that all extracts 
give curves of similar shape and having points of maximum at the 
relatively low concentrations ordinarily used in practice. Thomas and 


VEGETABLE TANNING 261 


Kelly showed definitely that the rise and fall in the curves cannot be 
attributed to variations in hydrogen-ion concentration, but is due to 
the increasing concentration of the other constitutents of the tan liquors. 


QUEBRACHO 3 QUEBRACHO 
(18.3 grams per liter) ; 18.5 g. per liter 


e 
24 hours 


6 hours 


HEMLOCK BARK HEMLOCK BARK 
(24 grams per liter) (80 grams per liter) 


21.5 weeks 

$$ *&—_*— 
2 weeks 

—G—__9-—_._-o— 


J day 24 hours 


LARCH BARK LARCH BARK 
(49 grams per liter) (90 grams per liter) 


Grams Tannin Combined with 100 Grams Hide Substance 


24 hours 


24 hours 


foeee 6 8. 10° 12 2a. An 6) eee LOa Le 
pH Value of Tan Liquor 
Fic. 121.—Rate of Tanning as a Function of pH Value. 


One explanation given for the appearance of points of maximum 
in the curves is that the rate of combination of tannin and hide sub- 
stance increases so rapidly, with increasing concentration of tan liquor, 


262 THE CHEMISTRY OF LEATHER MANUFACTURE 

that it soon reaches a point where the surfaces of the hide fibers quickly 
become so heavily tanned that they are rendered less permeable to 
the tannin remaining in solution. The interior of the fibers are thus 
prevented from tanning so rapidly, which accounts for the smaller 
amount of tannin fixed by the hide powder in the stronger solu- 
tions. Another explanation is furnished by the work of Thomas and 
Foster,° who observed that the electrical difference of potential at 
the surface of tannin particles decreases with increasing concentration 
of tan liquor. This would lessen the attraction between the tannin 
particles and the protein jelly and thus cause a decrease in the rate 
of combination. This seems the more probable explanation because 
a greater rate of diffusion of tan liquor into skin is obtained in 
practice by using more concentrated solutions. The curves represent 
the resultant of two effects: the increasing concentration of tannin 
tends to cause an increase in the rate of tanning and the increasing 
concentration of nontannin tends to cause a decrease in the rate of 
tanning. The point of maximum represents the point at which the 
effect of the increasing concentration of nontannin becomes greater 
than that of the tannin. 

These curves are in agreement with the findings of a number of 
investigators that highly concentrated tan liquors are very much less 
astringent than those of moderate concentrations. In practice, the 
degrees of astringency of tan liquors seem to follow curves similar 
to those in the figures. The use of concentrated liquors in tanning 
has been suggested by Seymour-Jones* and by Enna,® but the idea 
seems not to have been widely adopted, probably because it introduces 
complications in the later processes not easily overcome without some 
loss in quality of the finished leather. 


Rate of Tanning as a Function of pH Value. 


Thomas and Kelly® next turned their attention to the effect of 
the pH value of tan liquors upon the fixation of tannin by hide 
substance. The procedure adopted was the same as in the studies of 
the effect of concentration. In each case the pH value of the tan 
liquor, as determined by the hydrogen electrode, was adjusted to the 
desired value by the addition of sodium hydroxide or hydrochloric 
acid, Figs. 120 and 121 show the effect of change of pH value on 
the rate of the tanning of hide powder by solutions of quebracho, 
gambier, oak bark, wattle bark, hemlock bark, and larch bark ex. 
tracts. The curves contain a mine of information that requires careful 
study. 

The most elaborate set of curves is that for hemlock bark exiract, 
8 Ind. Eng Chose eed eae Tanning Extracts. A. W. Thomas and S. B. Foster, 
. erates. Tanning of Sole Leather. Alfred Seymour-Jones. J. Soc. Leather Trades Chem. 

® Rapid Tannage. Fini Enna. Ibid., 1 (1917), 36. 

®The Hydrogen-Ion and Time Factors in the Fixation of Tannins by Hide Substance. 


A. W. Thomas and M. W. Kelly. Ind. Eng. Che ‘ ; Di i i 
Kelly, CokumbisiUaree ress g tem. (1923); Dissertation, Miss Margaret W. 


VEGETABLE TANNING =. 263 


which may be discussed as typical. In the concentration experiments, 
a tan liquor containing 24 grams of solid matter per liter gave a much 
greater rate of tanning than one containing 80 grams per liter, but 
the curves in Fig. 121 show that this is dependent upon the pH 
value; at pH = 5, the more dilute solution tans at the greater rate, 
while at 2 and at 8, the more concentrated solution tans at the greater 
rate. 

In tanning for 24 hours, there is a steep rise in all curves to the 
left of pH = 5, which is exactly what one would expect, knowing 
that the positive electrical charge on collagen increases as the pH 
value falls from the isoelectric point and that the tannins are nega- 
tively charged at pH values higher than 2. In some cases a falling 
off in rate of tanning as the pH value drops below 2 is noticeable, 
but it must be remembered that the great tendency for collagen to 
swell and to hydrolyze at high acidities makes it difficult to get reliable 
data at pH values as low as 2. 

The most curious parts of the curves are those bétween the pH 
values 5 and 8. Since tannin particles are negatively charged in this 
region, the question that naturally arises is the possibility that the 
collagen may become increasingly positive with rise of pH value from 
5 to about 8. This might seem an absurd view were it not for the 
two points of minimum plumping of calf skin found by Wilson and 
Gallun and shown in Fig. 73 of Chapter 9. Here it was suggested 
that collagen undergoes a change of form, possibly an internal re- 
arrangement, in passing from an acid to an alkaline solution and that 
the two points of minimum, at pH = 5.0 andsat pli t="7.7, 1epresent 
the isoelectric points of the two forms. We may refer to collagen 
stable in acid solution as form A and collagen stable in alkaline solu- 
tion as form B. As the pH value is increased from 5.0 to 7.7, if 
the conversion of form A into form B proceeds at a greater rate than 
the formation of negatively charged ions of form A, then we should ex- 
pect the net charge on the collagen structure to become increasingly 
positive, which would result in an increased rate of tanning. 

The question was raised in discussion as to whether any fixation 
of tannin actually took place at pH values below 2 and above S2ein 
all of the experiments described, the powders were washed with dis- 
stilled water immediately after being taken from the tan liquor. Dis- 
tilled water usually has a pH value of about 5.8, due to dissolved 
carbonic acid, and this would tend to make the pH value of the solution 
absorbed by the collagen jelly approach the value 5.8 before it was 
all washed out and the observed fixation of tanning might have occurred 
during the washing rather than during the shaking with tan liquor. 
Thomas and Kelly showed, however, that fixation actually does take 
place at pH values below 2 and above 8. 

They prepared a solution of wattle bark extract containing 40 
grams of solid matter per liter and hydrochloric acid to bring the pH 
value to 0.87. Four portions of hide powder were tanned with this 
solution for 24 hours in the prescribed manner and then two were 


264 THE CHEMISTRY OF LEATHER MANUFACTURE 


washed with distilled water and two with a hydrochloric acid solu- 
tion having a pH value of 0.87 until no more tannin could be ex- 
tracted. The latter two were then washed free from hydrochloric 
acid with distilled water. The two powders washed with the acid 
solution were found to contain an average of 0.739 gram tannin com- 
bined with the original 2 grams of hide powder against 0.987 gram tan- 
nin for the powders washed with distilled water. This shows that, al- 
though washing with distilled water causes an increase in combined 
tannin found, there is actually a fixation of tanning taking place at 
pHi'0.87; 


Tanned Powders Washed with 
Distilled Water 


Tanned Powders Washed with 


A Buffer Solution of Same pH 
Value as Tan Liquor 


to is*] 
Oo oO 


Grams Tannin Fixed by 100 Grams Hide in 24 Hours 
h 
oO 


a 2 3 4 5 6 7 8 ¢ 10 


pH Value of Tan Liquor 


Fic. 122.—Showing the Rate of Tanning as a Function of pH Value and also 


the Effect of Washing the Tanned Powders with Buffer Solutions having 


the Same pH Value as the Tan Liquor Used in Tanning. 


They then prepared two series of solutions of hemlock bark ex- 
tract, containing 24 grams of solid matter per liter and having pH 
values ranging from 1 to 10, as determined by the hydrogen electrode. 
Portions of hide powder were tanned in each series for 24 hours 
in the prescribed manner and then the powders of one series were 
washed with distilled water, while those of the other were washed 
free from soluble tannin with solutions having the same pH values 
as the liquors in which the powders were tanned. For pH values 
of 4 or less, the solutions used for washing contained only hydro- 
chloric acid; for pH values from 5 to 9, they were made from M/rs 
sodium phosphate adjusted to the desired pH value with HCl or N aOH ; 
for pH = 10, a solution of sodium hydroxide was used. The final 
washing was done with distilled water. The results are shown in 


a SS an 


VEGETABLE TANNING 265 


Fig. 122. The two curves are not identical, but show plainly that 
tannin combines with hide substance at all pH values from I to Io. 
Where a buffer solution was used to wash the hide powder tanned 
at pH = 5, a greater fixation of tannin occurred. Salts at low con- 
centration have the property of increasing the fixation of tannin at 
pH = 5, as will be shown later. 


Stability of the Collagen-Tannin Compound. at Different pH Values. 


While studying the action of solutions of acid and alkali upon leather 
previously freed from water soluble matter, Wilson and Kern* 
found that tannin was extracted by dilute solutions of alkali, but not 
of acid. In an attempt to locate the pH value at which the collagen- 
tannin compound begins to hydrolyze, they performed the following 
experiment. A large amount of purified hide powder was tanned with 
quebracho extract at a pH value of 4.6, washed free from all soluble 
matter with distilled water, and then dried. Seven large reservoirs 
of buffer solutions were prepared by making up solutions of tenth- 
molar phosphoric acid with sodium hydroxide to produce the pH values 
5, 6, 7, 8, 9, 10, and 11, respectively. Eight-gram portions of the tanned 
powder were put into Wilson-Kern extractors ™ and extracted with 
4 liters of buffer solution, taking just 6 hours for all of the solution 
to percolate through the tanned powder. Each portion was extracted 
with a solution of different pH value. The extracted powders were 
washed free from buffer solution with distilled water and were then 
dried and analyzed for comparison with the original powder. All 
extracts were brought to a pH value of 4 and then tested for tannin 
with the gelatin-salt reagent. The buffer solutions extracted only 
negligible amounts of nitrogen from the powders. The results are 
shown in Table XXXII. 


TABLE XXXII. 


ANALYSES OF TANNED Hine Powper BErorE AND AFTER WASHING WITH 
SoLUTIONS OF DIFFERENT pH VALUES. 


Before After washing with solution of pH= 
washing 5 6 vk 8 9) 10 II 


mR ey aa se nw ees fe 0.2 0.4 0.5 0.4 0.6 0.3 0.5 0.4 
Hide substance (N x 5.62).. 84.2 83.9 83.9 83.9 84.2 847 84.9 85.3 
Tannin (by difference)..... 15.6 15.7 15.6 15.7 15.2 15.0 14,60) Gi43 
Per cent of total tannin ex- 

oo Lo rer none none none 2.6 3.8 6.4 8.3 
Test for tannin in extract...... neg. neg. meg. pos. pos. pos. pos. 


Leather tanned at pH = 4.6 is apparently resistant to hydrolysis 
by solutions having pH values up to some point between 7 and 8, 
but is at least partially hydrolyzed, and with increasing speed, as the 


10 Stability of the Hide-Tannin Compound at Different pH Values. J. A. Wilson and 

“i J. Kern. Presented before the Leather Division of the American Chemical Society, Sept. 
1922, 

* “11 For description, see J. Ind. Eng. Chem. 13 (1921), 772. 


2066 THE CHEMISTRY OF LEATHER MANUFACTURE 


pH value is increased above 8. This adds some weight to the sug- 
gestion that 7.7 represents the isoelectric point of one form of collagen. 
But, taken in conjunction with the finding of Thomas and Kelly that 
collagen and tannin form stable compounds at pH values greater than 
8, it also supports their view that the collagen-tannin compound formed 
in alkaline solution is different from that formed in acid solution, which 
will be made clearer when their later experiments are described. 


Effect of Neutral Salts upon the Rate of Tanning. 


The effect of the concentration of sodium chloride or sulfate upon 
the rate of tanning of hide powder by solutions of gambier and hem- 


GAMBIER (sodium chloride) GAMBIER 


(sodium sulfate) 


~so1,0-M salt 
eo 0 2.0-M salt 


HEMLOCK BARK 
HEMLOCK BARK 


(sodium sulfate) 


(sodium chloride) 


‘© 0.5-M salt no salt 


——o1.0-M salt 


“9 2.0-M salt 


Grams Tannin Fixed by 100 Grams Hide Substance 
Grams Tannin Fixed by 100 Grams Hide Substance 


go. -8--—--9 1,5-M salt 


2 5 8 
pH Value of Tan Liquor Before pH Value of Tan Liquor Before 
Adding Sodium Chloride Adding Sodium Sulfate 


Fic, 123.—Effect of Sodium Chloride Fic. 124—Effect of Sodium Sulfate 
and pH Value upon the Rate of and pH Value upon the Rate of 
Tanning. Time, 24 hours. Tanning. Time, 24 hours. 


lock extracts, at different pH values, has recently been studied by 
Thomas and Kelly.'’* In each test, 100 cubic centimeters of tan liquor, 
a weighed amount of salt, and the equivalent of 2 grams of water-free 
hide powder were put into a bottle and shaken, in a rotating box, 
for 24 hours. The contents were then transferred to a Wilson-Kern 
extractor, filtered, and washed until the washings gave no coloration 
with ferric chloride solution. The tanned powders were then dried 
in a vacuum at 100° C. and weighed, the increase in weight of the 
dry powder being taken as tannin fixed. 


2 The Influence of Neutral Salts upon the Fixation of Tannins by Hide Substance. 
A. W. Thomas and M, W. Kelly. Ind. Eng. Chem. (1923); (advance copy). 


VEGETABLE TANNING 267 


In order to guard against including as fixed tannin any matters 
rendered insoluble by the added salt, blanks were run leaving out the 
hide powder and corrections were made where necessary. 

The insoluble matter of the extracts was first removed by centri- 
fuging strong solutions, which were then diluted to contain 40 grams 
of solid matter of the tanning extract per liter, after adjusting the 
pH value to 2, 5, or 8, by addition of hydrochloric acid or sodium 
hydroxide. 

The effect of sodium chloride is shown in Fig. 123 and that of 
sodium sulfate in Fig. 124. At pH =2, both salts retard tanning 
to a very considerable extent, although sodium sulfate is always much 
more effective in this respect than sodium chloride. In each case the 
extent of the retardation is greater the higher the concentration of 
salt. At pH =8, the action of the salts is similar, but less pro- 
nounced. At pH = 5, the action is still less pronounced and is even 
reversed by concentrations of sodium chloride less than twice molar, 
which seem to cause an increase in rate of fixation of tannin. 

The marked reduction in the rate of tanning at pH = 2 exerted 
by the salts is probably due primarily to the reduction of the electrical 
differences of potential between the collagen jelly and the liquor on 
the one hand and between the liquor and the surface film surrounding 
the tannin particles on the other. The potential difference between 
collagen jelly and liquor is probably at its maximum value in the 
vicinity of pH = 2 and, consequently, the depressing action of salt 
should be greatest at this point. According to the Procter-Wilson 
theory of tanning, a diminution in this potential difference must result 
in a decrease in rate of tanning. The greater effect of sodium sulfate 
may be attributed to the divalent sulfate ion, as explained in Chapter 5. 

With the decreasing potential difference between the liquor and 
the surface film of solution in contact with the tannin particles, there 
would be an increasing tendency for the tannin particles to form 
aggregates and finally to precipitate out, further decreasing the rate 
of combination of tannin with collagen. 

The effect of hydration of the added salt is to remove water from 
the role of solvent, as explained in Chapter 4, and this would cause 
a virtual increase in concentration of tannin. Thomas and Kelly point 
out that opposed to this, within certain limits, would be the tendency 
for the salt to cause an aggregation of the particles of tannin. These 
opposing actions may explain the behavior of sodium chloride at 
pH=s5. At this point the potential difference between the collagen 
jelly and the liquor would be near its minimum value and hence would 
be but little affected by the salt. The effects of hydration and of 
aggregation would therefore be much more pronounced at this point, 
and Thomas and Kelly suggest that the increase in rate of tanning 
by molar and half-molar sodium chloride may be due to the hydration 
effect and the decrease in rate of tanning by the stronger sodium 
chloride solution and the sodium sulfate solutions to the aggregation 
factor. They are continuing their studies of the action of salts upon 
the vegetable tanning process. 


268 THE CHEMISTRY OF LEATHER MANUFACTURE 


Degree of Plumping of Skin as a Function of Concentration of 
Acid and Salt in Tan Liquors. 


Tanners of heavy leathers usually attach much importance to the 
degree to which the skin is swollen, or plumped, during the tanning 
operation. It is generally assumed that greater yields of leather are 
obtained when the skin is tanned in a highly plumped condition. If | 
the plumping by means of acid is carried to excess, however, the skins 
will be ruined. The first sign of danger in this direction is a wrinkling 
and reticulation of the grain surface of the skin. A rapid tanning of 
the surfaces of the skin follows, rendering them almost impermeable to 
the tannin remaining in solution, and the fibers in the interior remain 
taw and swell considerably, assuming a glassy appearance. If left 
long in this condition, especially in warm liquors, the collagen hydrolyzes 
and the skin is damaged beyond hope of recovery. 


TABLE XXXIII. 


DEGREE OF PLUMPING OF CALF SKIN Propucep BY TAN Liguor CONTAINING 25 
GRAMS OF OAK BARK ExtTrRAcT PER LITER AND Lactic ACID AND 
SopIUM CHLORIDE AS SHOWN IN THE TABLE. 


Moles per Liter Gauge Readings in MM. Final 
Lactic Sodium (average of triplicates ) pH Value 
acid chloride Initial Final Ratio *.. “at 26°U 
None None 1.346 2.150 1.60 4.63 
0.0025 bi 1.411 2.343 1.66 3.904 
0.0050 ‘ 1.383 2.699 205 3.74 
0.010 m 1.433 3.842 2.68 3.47 
0.025 a 1.470 4.504 3.10 3.05 
0.050 “a 1.360 4.497 3.31 2.81 
0.100 - 1.434 5.100 3.56 ee 
By 0.05 1.456 4.522 3.11 2.49 
. 0.10 1.458 3.918 ~ 2.69 2.47 
0.25 1.461 3.483 2.38 2.43 
2 0.50 1.420 2.182 1.54 2.37 


* This is a measure of the degree of plumping. 


Wilson and Gallun** studied the-effect of acids and salts upon 
the plumping of calf skin in tan liquors, using their method, which 
is described in Chapter 8. The effect of lactic acid and of sodium 
chloride upon the degree of plumping of calf skin in a solution of oak 
bark extract is shown in Table XXXIII and in Figs. 125 and 126. 

For this experiment a piece was selected from the butt of a calf 
skin, after liming, unhairing, and washing, of as nearly uniform thick- 
ness as possible and cut into squares having a side of about 2 centi- 
meters. These were delimed by washing with several changes of 
0.o1-molar hydrochloric acid containing 10 per cent of sodium chloride, 
then kept over night in a saturated solution of sodium bicarbonate 
containing 10 per cent of sodium chloride, washed thoroughly, and 


18 Direct Determination of the Plumping Power of Tan Liquors. J. A. Wilson and 
A. F, Gallun, Jr. Ind. Eng. Chem. 15 (1923), 376. 


VEGETABLE TANNING 269 


finally bated for 5 hours at 40° C. ina solution of 1 gram per liter 
of pancreatin, having a pH value of 7.6. The pieces were then washed 
for 24 hours in running tap water and were kept under distilled water 
in a refrigerator at 7° C. until used. The resistance to compression 
of each piece of skin was measured by means of a Randall & Stickney 
thickness gauge with a flat, metal base, upon which the piece of skin 
was placed, and a plunger, having a circular base I square centimeter 
in area, capable of pressing on the surface of the skin under constant 
pressure. The gauge reading was taken, in every case, exactly two 
minutes after dropping the plunger onto the skin. 


3.54 OAK BARK 
(lactic Acid) 


OAK BARK 


(lactic acid) 


Degree of Plumping 


(final/initial gauge reading) 
Degree of Plumping 


(final/initial gauge reading) 
~ 
a 


eepeoeOr o.0 4,0 4.5 O10 0,8 (016 Ok 0.5 
pH Value of Tan Liquor Moles Sodium Chloride per Liter 
Fic. 125.—Effect of pH Value of Tan Fc. 126.—Effect of Sodium Chloride 
Liquor upon Degree of Plumping upon Degree of Plumping of Calf 
of ‘Calf Skin. Skin in Tan Liquor Acidified with 


Lactic Acid. 


7 Eleven tan liquors were prepared as indicated in Table XXXII. 

The gauge readings of pieces of the standard skin were taken and they 
were then shaken with water to bring them back to their normal shape, 
after being compressed in the gauge. They were then put into the tan 
liquors and allowed to remain there for 24 hours at 20°C. The final 
eauge readings were then taken. In each case 3 pieces of skin were 
put into 100 cubic centimeters of tan liquor and the agreement be- 
tween the triplicate determinations was satisfactory. The degree of 
plumping caused by the liquor is measured by the ratio of the final 
to the initial gauge reading. 

The actions of the acid and the salt are not exactly the same as 
they would be in pure water, but are complicated by the tanning action 
of the liquor, which decreases the swelling power of the skin. The 
general tendency of the acid, nevertheless, is to swell the skin and the 
action of the salt to counteract this swelling. 

McLaughlin and Porter ** made a rather interesting study of the 
change in weight of limed steer hide during immersion in tan liquors 


4 On the Swelling and Falling of White Hide in Vegetable Tan Liquors, G. D. 
McLaughlin and. R. E. Porter, J. Am. Leather Chem. Assoc. 15 (1920), 557. 


270 THE CHEMISTRY OF LEATHER MANUFACTURE 


of various compositions. Unfortunately the pH values of the liquors 
were not determined and, in many cases, it is not clear how much of 
the change observed is due to variation of hydrogen-ion concentration. 


Rapid Tannages. 


Numerous accounts appear in the literature of attempts to hasten 
the tanning process, especially for heavy leathers. But few of these 
have yet developed to a point where the mechanism of the process 
is well defined. In many cases, it appears likely that the added ac- 
celerator acts only indirectly by bringing about a more favorable reaction 
of the tan liquor itself. 

One process for hastening tanning that seems, on the face of it, 
to merit further investigation is that described by Cross, Greenwood, 
and Lamb.’* In the course of investigations on the hemi-celluloses of 
seed endosperms, the authors studied their compounds with tannin, 
which may be made to form apparently homogeneous jellies. From 
previous experience in the dyeing of silk, the authors conceived the 
idea of controlling the astringency of the tannin by using it in the 
form of a compound with the hemi-cellulose. They found that the 
use of “gum tragasol” in conjunction with the tannin solution caused 
a very rapid penetration of tannin into the skin. Complete penetra- 
tion of very thick hides was obtained in two or three days, although 
the reduced rate of combination between collagen and tannin required 
the keeping of the hides in the liquor for a somewhat longer time than 
this. 

A similar type of process is that proposed by Turnbull and Car- 
michael *° in which the tanning materials are dissolved in a jelly 
formed of a starch solution. 

Another process intended to hasten the tanning of heavy hides 
is that of C. W. Nance?” and known as the vacuum process. The 
hides are put into a tank, which is then evacuated to a pressure of 
0.5 lb. per square inch. The temperature is then gradually raised 
to the point at which water boils at this pressure. The tan liquor is 
introduced and the temperature allowed to fall slowly to permit the 
hide to absorb tan liquor to replace the water lost by boiling. By 
a proper regulation of temperature and pressure as well as concentra- 
tion of tan liquor, it is claimed that an enormous reduction in time 
of tanning can be effected. ; 

Attempts have been made at various times to hasten the tanning 
process by the application of an electric current. The hides are placed 
between. carbon electrodes and the current turned on; in moving towards 
the anode, the tannins are thus made to penetrate the hide. Ridea] 
and Evans 18 pointed out that to get good results the conductivity of 
and ta ae Bos Colorin uthlauegs: © F: Crm G. Vs Greenwood 

re e pate Serer i ss Poe rete es - British Pat. 110,470, Feb, 24, 3O17. 


3 
7 Some Experiments on the Theory of Electro-Tanning, <. Ri : 
pe ee ches 2 sattisraeoe y nning. E. K. Rideal and U. R. Evans. 


VEGETABLE TANNING 271 


the liquor must be very low and that the cathodes should be made 
of carbon and the anodes of copper. Williams *® found that a direct 
current causes a rapid destruction of pure gallotannic acid, which did 
not take place when an alternating current was used. Following the 
presentation of Rideal and Evans’ paper, J. G. Parker said that he 
had experimented with electrical tanning and doubted that it had any 
advantages over systems not involving the use of the electric current. 
At any rate, it has not been adopted very widely as yet. 

Much attention has been paid recently to the effect of adding 
organic compounds containing sulfonic groups to vegetable tan liquors 
upon the rate of penetration of the tan liquor into the hide. Among 
the materials commonly used may be mentioned the lignosulfonic acids 
obtained from the so-called sulfite cellulose, a by-product in the manu- 
facture of paper from wood pulp, and also the synthetic products 
known as syntans, discovered by Stiasny, which will be discussed in 
Chapter 15. These materials act much like certain nontannins naturally 
occurring in vegetable tanning materials in lessening the astringency 
of the liquors and hastening the penetration of tannin into the skin. 

Apparently they have lower molecular weights than the tannins, 
which enable them to diffuse into the skin more rapidly. Since they 
actually combine with the collagen, they retard the combination of the 
true tannins with collagen, which thus permits tannin to diffuse into 
the interior that would otherwise have combined with collagen at the 
outer surface. This makes them valuable materials to use in the early 
stages of tanning. Whether or not the sulfonic groups which they 
possess are harmful for some kinds of vegetable tanned leathers has 
been the subject of debate, but has not yet been clearly settled. 

The acid character of these sulfonic groups gives the liquors a very 
low pH value, which in turn causes a lightening of the color of both 
the liquors and the leather. In some cases there seem to be combina- 
tions between the tannins and the sulfonic compounds, resulting in com- 
pounds less easily precipitable than the original tannins. The synthetic 
materials seem also to cause a reduction of some of the more highly 
oxidized tannins, which may explain in part the lesser tendency of 
certain mixtures to precipitate upon the addition of acid. 


Theory of Tanning. 


Until it became possible to treat the chemistry of the proteins in 
a quantitative manner, there was little hope of developing a quantita- 
tive theory of tanning. Numerous attempts to determine the relative 
combining weights of gelatin and tannin led only to variable and often 
apparently contradictory results because of the failure to appreciate 
the existence of uncontrolled variable factors. A review of the older 
Set on theories of tanning would be of little more than historical 
value. 

The modern theories of tanning are following the general trend of 


% Inquiry into Electrical Tannage. O. J, Williams, Collegium (1913), 76. 


aye THE CHEMISTRY OF LEATHER MANUFACTURE 


development of the chemistry of the proteins. One school of thought 
treats the theory of tanning from the viewpoint of the physical chem- 
istry of the proteins and the other from that of organic chemistry. 


Procter-Wilson Theory. 


The line of investigation of the physical chemistry of the proteins 
started by Procter led naturally to the conception of the mechanism 
of tanning formulated by Procter and Wilson.2® The work leading 
to the formulation of this theory is given in detail in Chapter 5 and 
need not be repeated here. When in equilibrium with a tan liquor 
having a pH value lying in the range 2 to 5, collagen may be looked 
upon as constituting an aggregate of complex cations balanced by 
much simpler anions held in the solution immediately in contact with 
the collagen structure by the same forces that hold all oppositely charged 
ions together. We may assume that the collagen composing a hide 
fiber has a structure corresponding to that of gelatin when set to a jelly. 

The theory may be pictured very simply by considering a piece of 
skin in contact with a solution containing only tannin and the acid HA. 
When equilibrium is established between the collagen and the acid, in 
the tan liquor let 

x = [H*] = [A’] 


and in the jelly phase of the collagen let 


Vea 
and z—= [CH*] (ie., concentration of collagen cation) 
whence (Aaa yg | 


The equilibrium conditions are exactly analogous to those described 
for gelatin, from which it is apparent that there will be an electrical 
difference of potential between the jelly phase and the external solution 
expressible quantitatively by : 
rope as log — RY oe oe VA 
- — x inne 2x 

Each tannin particle is negatively charged and, consequently, must 
have associated with it an equivalent number of cations held in the 
solution immediately in contact with the particle, which we may call 
the surface film for convenience, although it makes no difference to 
the theory whether the tannin particle is solid, like a gold particle, or 
a jelly particle capable of absorbing solution. Let the concentration 
of these cations be represented by z, and the concentration of the 
anion A’ in the surface film by y,; the total concentration of cation 
then equals y, + 2,. The electrical difference of potential between the 
surface film and bulk of solution then equals 


ee of Vegetable Tanning. H. R. Procter and Je As Bisco? J, Chem. Soc, 109 


VEGETABLE TANNING 273 


eat 2 ae eal & 2% 
E, = —— log — os —— log ee? 
peeia OV dae fey 


F yi F 
It is evident that E and FE, are of opposite sign. 

According to the Procter-Wilson theory, the first important action 
in the mechanism of tanning results from the tendency for EF and FE, 
to neutralize each other. The initial rate of tanning will, therefore, 
be measured by the sum of the absolute values of the potential 
differences, or 


7 a! ll ES aN 
Bo (24+ V4 +27) (Ha -+ 4x? F217) 


In this expression, z is measured by the absolute value of the electrical 
charge on the collagen and 2, that on the tannin particles, while x 
represents the hydrogen-ion concentration of the tan liquor. For a 
fixed value of x, an increase in value of either zg or g, evidently causes 
an increase in the rate of tanning. 

_ Now, if we introduce a salt, say sodium chloride, and let its con- 
centration in the tan liquor at equilibrium be represented by 


u = [Na*] = [Cl’], 


from the reasoning in Chapter 5, we see that the initial rate of tanning 
is now determined by the expression 


RT tog Ceemen ste 40X52 
F eee 40x 1-0)? 2") [—z, + V4(x + u)? +2,7] 


It is apparent that an increase in u, provided it does not increase z or 4, 
will cause a decrease in rate of tanning. This explains, in part, the 
retarding effect of salts upon the rate of tanning. If u is increased 
without limit, the value of the above expression becomes zero. 

When the surface film surrounding the tannin particle has joined 
the solution constituting the jelly phase of the collagen and thus neu- 
tralized the potential difference which each had against the external solu- 
tion, the actual charges on the collagen and tannin are free to neutralize 
, each other, as in the combination of any two oppositely charged ions 
which tend to form a slightly dissociated salt. 

Like the physical chemistry of the proteins, outlined in Chapter 5, 
this theory is capable of almost indefinite extension by mathematical 
treatment. Since the quantitative testing of the theory has only just 
been begun, such extensions may well be left for some future time. 
It is worthy of note, however, that the theory has proved a valuable 
guide in the development of tanning processes and no facts observed 
in tanning practice have yet been shown to be out of harmony with it. 

It is interesting to speculate on the probable combining ratio of 
collagen and pentadigalloyl glucose. Taking the author’s value of 750 
as the equivalent weight of collagen and assuming that each digalloyl 


274. THE CHEMISTRY OF LEATHER MANUFACTURE 


radical is capable of combining with collagen, we arrive at a combining 
ratio of 340 parts of tannin to 750 parts of collagen, or 45.3 per 100 
parts of collagen. It may be only a coincidence, but this ratio repre- 
sents the minimum possible for vegetable tanned leather to pass as 
fully tanned, at least in the author’s experience. On the other hand, 
when skins are allowed to remain in the tan liquors for months, the 
ratio approaches the value of 90 parts of fixed tannin per 100 of col- 
lagen, but the author has never known it to pass this value in practice. 
Of course it is appreciated that different tannins may have different 
molecular weights, which would cause some deviation in the ratio to 
be expected. Any supposition as to the combining proportions of col- 
lagen and tannin is admittedly highly speculative in view of our meagre 
knowledge of the mechanism of tanning, but where so little is known, 
such speculations are valuable in forming a nucleus from which to 
build. 

The Procter-Wilson theory does not concern itself with the con- 
stitutions of the collagen cation and tannin anion, nor does it deal 
with possible combinations of collagen and tannin where these have 
electrical charges of the same sign, a condition which rarely, if ever, 
occurs in tanning practice. 

Thomas and Kelly 74 have recently started an investigation to de- 
termine the nature of the combination of collagen and tannin at 
different pH values. Trunkel ?* had previously shown that the water- 
insoluble compound of gelatin and tannin can be resolved into its 
components by digesting with ethyl alcohol, provided the digestion is 
carried out before the precipitate has dried. After drying, the gelatin- 
tannin compound is unaffected by alcoholic digestion. Thomas and 
Kelly studied the effect of alcohol upon collagen tanned at different 
pH values. 

Tan liquors were prepared having pH values of 1, 3, 5, 7, and 9. 
In the study of hemlock bark extract, portions of hide powder con- 
taining I gram of dry protein were shaken for 24 hours, at room 
temperature, with 50 cubic centimeters of tan liquor containing 2.7 
grams of solid matter of the hemlock extract. The tanned powders 
were then filtered and washed in Wilson-Kern extractors until the 
wash water gave no color upon addition of ferric chloride. The wet 
powders were then transferred to Thorn extractors and extracted with 
g5-per cent alcohol. In this type of extractor, the material is ex- 
tracted by the hot vapors as well as by the condensed solvent. 

At intervals the alcoholic extracts were transferred to beakers, 
evaporated to dryness, dried for 4 hours im vacuo at 100° C. and 
weighed. After apparently complete extraction, the tanned powders 
also were dried im vacuo and weighed, the loss of tannin due to the 
alcohol extraction being calculated by comparison with a control series 
not treated with alcohol. 


21 The Difference in Kind or Degree of Tannin Fixation as a Function of the Hydrogen- 
Ion agape, ec A. W. Thomas and M. W. Kelly. Ind. Eng. Chem. (1923); (advance 
copy). ot 
‘#2 Gelatine and Tannin. H. Trunkel. Biochem, Z, 26 (1930), 458, 


Se 


VEGETABLE TANNING 2475 


Table XXXIV shows the results obtained from weighing the residues 
from the alcoholic extracts and Table XXXV those obtained from 
the dry weights of the extracted leathers. Apparently alcohol de- 
composes most easily those leathers which were tanned at pH values 
lying between 3 and 5, the region in which tanning is usually done 
in practice. It is also apparent that leathers tanned at pH values 
greater than 5 are much more resistant to decomposition than those 
tanned at values less than 5. Table XXXV also shows the effect 
of extracting previously dried leathers with alcohol; drying evidently 
brings about a more permanent fixation of the tannin. 


TABLE XXXIV. 
ExtTRACTION BY ALCOHOL OF FIXED TANNIN FROM LEATHERS TANNED WITH 
Hemiock Bark Extract AT DIFFERENT pH VALuges. Ficures OBTAINED 
BY WEIGHING THE Dry RESIDUES FROM THE ALCOHOLIC EXTRACTS. 


Per cent of Total Fixed Tannin Removed 


Tanned at pH by Extraction for 
Value of 1 hour 45 hours gt hours 
les nts tina cs so 6.1 19.3 23.3 
On a 77 24.1 28.9 
MT eee ko ss oak x 7.8 17.1 22.0 
an Ree an 3.1 6.9 9.8 
Cl oo a 4.2 6.3 8.4 


TABLE XXXV. 


(Same Experiment as Described in Table XXXIV. But the figures in this table 
were obtained by weighing the dry leather after the alcohol extraction.) 


Per cent of Total Fixed Tannin Removed 


Tanned at pH by 91 Hours’ Extraction of the 
Value of Wet leather Dried leather 
Moy a lk oc 0% 16.6 0.0 
Se chee o's where ois'.5 > ae 23.3 2.8 
ct Se ian, Pollo ey ae oa 25.1 4.4 
Oo) Aa eee ae 4.6 0.6 
a ee eee 0.6 0.0 


A curious finding is that the figures in Table XXXV show a smaller 
loss of tannin than those in Table XXXIV. Some light is thrown 
upon this difference by a series of experiments with gambier. These 
were similar to the hemlock series except for the fact that the 50- 
cubic centimeter portions of tan liquor contained 2 grams of dry 
gambier solids. Table XXXVI shows the results obtained by weigh- 
ing the dry leathers after extraction with alcohol. The leather tanned 
at a pH value of 9 actually shows a gain in weight upon extraction with 
alcohol. Thomas and Kelly suggest the hypothesis that this gain may 
be due to an oxidation of the alcohol to aldehyde followed by an 


aldehyde tannage. Tan liquors absorb oxygen readily at a pH value 


of 9 and darken in color as oxidation proceeds. The author has found | 


276 THE CHEMISTRY OF LEATHER MANUFACTURE 


that oxidized tannins present in leather cause an oxidation of un- 
saturated oils used in fatliquoring leather, as determined by the ratio 
of oxidized to unoxidized fatty acids subsequently extracted from the 
leather. It is therefore not unreasonable to suppose that tannins which 
have been oxidized at a pH value of 9 may be able to bring about 
an oxidation of the alcohol molecule. 


TABLE XXXVI. 


EXTRACTION By ALCOHOL OF FIXED TANNIN FROM LEATHERS TANNED WITH 
GAMBIER EXTRACT AT DIFFERENT pH VALUES. FIGURES OBTAINED BY 
WEIGHING Dry LEATHERS AFTER THE ALCOHOL EXTRACTION. 


Per cent of Total Fixed 
Tannin Removed by Ex- 


Tanned at pH traction for 91 Hours 
Value of of the Wet Leathers 
DV sileiel sueavejeteca hue’ elise s a ierets 17.5 
Pept, ih MR Ie tee gs 26.3 
ee Pe Dae ee ee? ee 19.5 
ec Oe Te 0.7 
ine See ere oe eee (Gain of 13.8 per cent!) 


The most important finding in this work is that the kind of 
fixation of tannin by collagen at pH values lower than 5 is different 
from that at pH values greater than 5. The simple theory of Procter 
and Wilson does not take into consideration the complex organic re- 
actions which apparently occur in tanning with liquors having a pH 
value greater than 5, nor the changes in the collagen-tannin compound 
which take place upon drying and aging. 


Oxidation Theory. 


Of the various theories of tanning treating the subject from the 
standpoint of organic chemistry, the oxidation theory supported by 
Meunier, Fahrion, and others is the only one meriting serious con- 
sideration. Meunier ** and his co-workers found that skin could be 
converted into leather by bringing*it into contact with a solution of 
benzoquinone. The color of the skin changed successively to light 
rose, to violet, and to brown. A leather of remarkable resistance to 
boiling water was obtained. An observation of great theoretical sig- 
nificance was that a portion of the quinone was reduced to quinol 
during the tanning action. Meunier concluded that part of the quinone 
had been reduced by the oxidation of the collagen and that only the 
oxidized collagen entered into combination with the remaining quinone. 
He likened the action to that of quinone upon aromatic amines: 


C,HsNH, + 2C,H,O2 = CsH,;N(C,H,O,) + C,H,(OH). ; 
2C.H;NH, + 3C,.H,O2 = (CsH,N),C,H,O, + 2C,H,(OH). 


_ % Modern Theories of the Various Methods of Tanning. L. Meunier. Chimie & indus. 
trie 1 (1918), 71, 272; English translation, J. Am. Leather Chem. Assoc, 13 (1918), 530, 


VEGETABLE TANNING 277 


Assuming the existence of primary amino groups in the collagen mole- 
cule, the compounds formed might be represented by the formulas: 


O R—N—O 
h—N= >C.H, | > CoH, 
O R—N—O 
Fahrion 24 suggested the following reactions: 
2RNH, + O=R—NH—NH—R-+H,.O 
R—NH R—NH—O 


In vegetable tanning materials, Meunier assumes that quinones are 
formed by oxidation and that it is these which react with collagen to 


form leather. 


Powarnin”® objected to the assumption of the formation of 
quinones by oxidation and suggested that they are formed by a. 
tautomeric change, thus 


CH CH 
ES aN 
fd Cee OT HC CH—O 
Peal = [ae*| | 
he, CC, OH HC CH—O 
ee Wee 
CH Cliss 
enol form keto form 


The enol form was supposed to be stable chiefly in alkaline solution 
and the keto form in acid solution. 

According to Powarnin’s view, only the keto form has tanning 
properties, the action being represented as follows: 


tannin protein leather 


The organic chemistry of vegetable tanning has not yet passed the 
stage of speculative hypotheses. 


24W. Fahrion. Mon. sci. (1911), 3613; (1914), 112. 
26 Active Carbonyl and Tannage with Organic Substances. G. Powarnin. 


(1914), 634. Collegium 


Chapter 14. 
Chrome Tanning. 


Although the tanning of skins by means of chromium salts is only 
of comparatively very recent origin, a large proportion of the world’s 
supply of light leathers is now tanned by this process. In 1858 Knapp? 
described a process for tanning skins with salts of aluminum, iron, and 
chromium, but chrome tanning did not come into prominence commer- 
cially until after the appearance of the patents of Augustus Schultz, 
of New York, in 1884. In Schultz’s process, the skins, after bating 
or deliming, were tumbled in a solution of potassium bichromate and 
hydrochloric acid until the chromate had completely penetrated the 
skins, after which they were allowed to drain. They were then tum- 
bled in a solution of sodium thiosulfate acidified with hydrochloric 
acid, which reduced the chromate to chromic salt, in which condition 
it combines with the skin protein, yielding a very stable leather. 

Schultz’s system of tanning is known as the two-bath process. In 
1893 Martin Dennis patented a system for tanning skins directly in a 
solution of basic chromium chloride along the lines suggested by Knapp 
in 1858. This one-bath process was naturally to be preferred to the 
two-bath process and soon gained precedence over it, except in the 
manufacture of glazed kid leathers, where the two-bath process seemed 
to yield a leather having more desirable properties. No explanation 
has yet been forthcoming as to why this should be so, although the 
author believes that the same results can be obtained by the one-bath 
process, if the reaction of the liquor is made to equal that of the liquor 
present in the skins during the second bath of the two-bath process. 
In the ordinary two-bath process, the acidification of the sodium thio-. 
sulfate causes a deposition of sulfur in the leather, which affects its 
properties, but a similar result can be obtained by the use of sodium 
thiosulfate in the one-bath process. On the other hand, the precipita- 
tion of sulfur in the two-bath process can be avoided by using sodium 
bisulfite as the reducing agent. Basic chromium sulfate was found 
to be superior to the chloride for one-bath tanning, as well as cheaper, 
and is now almost universally used. | 

Commercial one-bath chrome liquors are usually made from chrome 
alum or from sodium bichromate. A popular method is that devised 


1 Nature and Essential Character of the Tanning Process and of Leather. F.2K 
- Ener Buchhandlung (1858); English translation, J. Am, Leather Chem, Anoeiee 
1921), 658. 


278 


ee a a 


CHROME TANNING 279 


by Procter? in which acidified sodium bichromate solution is reduced 
to chromic salt by the addition of a solution of glucose. A great 
variety of reducing agents have since been suggested or patented for 
the purpose. A very convenient method, described independently by 
Balderston * and Procter,* consists in passing sulfur dioxide gas into a 
solution of sodium bichromate until the reduction is complete. The 
equation usually given for the reaction is as follows: 


Na,Cr,O; + BOO. +- H,O — Na.5O, + 2CrOHSO,, 


but this merely indicates the relative basicity of the final liquor, the 
end product probably being much more complex than this. 

In modern practice, it is customary to pickle the skins from the 
beamhouse before chrome tanning, as described in Chapter 10. This 
has the advantage of bringing all skins into a uniform condition. 
After pickling, the skins are put either into a drum or a paddle vat 
and tumbled or paddled with chrome liquor until completely tanned, 
which condition is determined by placing a cutting in boiling water. 
Any untanned portions are converted into gelatin and this causes the 
piece of skin to shrivel up and curl. When the skin is completely 


tanned, it is apparently entirely unaffected by boiling water. The rate 


of penetration of the chromium salts into the skin, the rate of tanning, 
and the properties of the resulting leather are markedly influenced by 
changes in concentration of chromium salt, neutral salt, and hydrogen 
ion. Conditions are made even more complex by the important effects 
of time, temperature, and degree of hydrolysis of the chromium salts. 
All of the variables must be adjusted to suit each other as well as the 
condition of the skin as it enters the tan liquor and the processes to 
which it is to be subjected after tanning. Probably no two tanneries 
operate exactly alike and very few would dare to deviate far from 
the practice found to give good results under their particular conditions 
of operation. . 


Chromium Collagenate. 


It now seems fairly well established that acid and basic radicals 
form definite salts with proteins. There is nothing novel about the 
assumption that chromium, or other metallic radical, can form with 
collagen a series of salts that might be called chromium collagenates. 
Moreover, such an assumption is a convenient one, even though it may 
be shown later that the compound formed is much more complex than 
would be indicated by the term chromium collagenate. 

If the author’s ® value of 750 be accepted for the equivalent weight 
of collagen, then the smallest amount of chromic oxide required to 


2H. R. Procter. Leather Trades Rev., Jan. 12, 1897. 

-§L, Balderston. Shoe & Leather Rep., Oct. 18, 1917. 

4H. R. Procter. J. Roy. Soc. Arts. 66 (1918), 747. 
( oa teas of Leather Chemistry. J. A. Wilson. J, Am. Leather Chem, Assoc, 12 
1917), 108. 


280 THE CHEMISTRY OF LEATHER MANUFACTURE 


convert I00 grams of collagen into the chromium salt would be 
(152x 100)/(6x 750), or 3.38 grams. Lamb and Harvey ® found 
that chrome leather showing less than 2.8 to 3.0 per cent of chromic 
oxide, based on the dry leather, was invariably undertanned. Based 
upon actual collagen, the figure would be about 3.4. That the figure 
3-38 has some significance will be made more apparent later. This 
value assumes that all of the three bonds of the chromium ion have 
entered into combination with the protein and we should expect such a 
compound to be extremely stable, which chrome leather undoubtedly is. 

The most exhaustive studies yet made of the combination of collagen 
and chromium at measured concentrations of hydrogen ion, chromic 
oxide, and neutral salt are those of Thomas and his collaborators, which 
will be given in some detail. 


Hydrolysis of Chromium Salts. 


Being a salt of a strong acid and a weak base, chromic sulfate 
hydrolyzes to a very considerable extent in aqueous solution, yielding 
free sulfuric acid and a series of 
basic chromic sulfates. Thomas and 
COMMERCIAL CHROME DIQUOR) Baldwin ® = fol ae change in 

immediately degree of hydrolysis of chromic 
salts, under various conditions, by 
measuring changes in hydrogen-ion 
concentration. Studies were made 
of solutions of C.P. chromic sulfate 
and chromic chloride and also of a 
typical commercial chrome liquor, 
which showed by analysis: Cr.Onz, 
14.3 per cent; Fe.O3, 1.9 per cent; 
immediately Al,Os, 0.2 per cent; SOs, 23.5 per 
cent; Cl, 0.2 per cent; and = ba= 
sicity corresponding to the formula 
Cr(OH)1.2(SOu)o.9, the sulfate 
present in excess of that called for 
by this formula being present as so- 


CHROMIC SULFATE 


PH Value of Chrome Liquor 
rw) es) 
a ° 


LO. 8720 “320 M46 dium sulphate. 
Grams Cro0z per Liter Effect of dilution: Fig. 127 
Fic. 127.—Effect of Dilution upon the shows the pH values of both chro- 


Hydrolysis of Chrome Liquors. mic sulfate and the commercial 
chrome liquor at different concen- 

trations. Strong solutions of each were diluted to increasing extents 
and the hydrogen-ion measurements were made immediately and also 
after the diluted solution had stood for 7 or 9 days. With both mate- 


®° Estimation of Chromic Oxide cs Chrome Tanned Leather. M. C. Lamb and A, Harvey. 
Collegium (London Edition) (1916), 201. ; 

The Acidity of Chrome Liquors. A. W. Thomas and M. E. Baldwin. J. Am. Leather 
Chem. Assoc. 13 (1918), 192. ‘ 

® Contrasting Effects of Chlorides and Sulfates on the Hydrogen-Ion Concentration of 
Acid Solutions. J, Am. Chem. Soc. 41 (1919), 1981. 


CHROME TANNING 281 


rials, the effect of dilution is naturally to raise the pH value, but, upon 
standing, the pH value of the commercial liquor continues to rise, while 
that of the chromic sulfate falls. 

‘Effect of added acid or alkali: Fig. 128 shows the effect of adding 
sulfuric acid or sodium hydroxide to solutions of the commercial chrome 
liquor and of pure chromic sulfate. In each case a given amount of 
concentrated chrome liquor was mixed with a definite volume of stand- 
ard sulfuric acid or sodium hydroxide and the mixture was diluted 
to 50 cubic centimeters. The hy- 


drogen-ion concentration was deter- COMMERCIAL 
mined immediately after mixing 4,0 ane 
and diluting and also after definite | 

. ; oe 3.5 

intervals of time. The addition of 

acid causes a corresponding increase 3.0 

in hydrogen-ion concentration, but 


_ this causes a repression of hydroly- 2.5 

sis of the chromium salt. The curves 
show that changes in degree of 
hydrolysis require a considerable 


8 
& 
4 
length of time; after the addition of B 4,5 
acid, the hydrogen-ion concentra- 4 eer: 
tion continues to fall for many days, « a SULFATE 
approaching, but never reaching the § 3,5 ot 
value it had before the addition of & 
the acid. & 3,0 ‘ 
The lowering of the hydrogen- & ae 
ion concentration by the addition of  7*® a 
alkali causes an increase in the de- 2,0 Shlemaetietats 
gree of hydrolysis of the chromium - a eee 


salt and the hydrogen-ion concen- 1B EL 
tration continues to rise towards the = | ~ _ | after 30 de; 
value it had before adding the a 
alkali. The long time required for 
such systems to reach equilibrium, 
after a disturbance, increases the 
difficulty of investigations of the BG «Add ed’ 0. 2M 1p 20¢00x ua0H 
chemistry of chrome tanning. og. c ae ee eee eee 
° rams Chromic Oxide 

Effect of neutral salts: In 5, 128 Rffect of Added Acid or 
Chapter 4 it was pointed out that Aitatin unde tain Ledeen ae 
the hydrogen-ion concentration of Chrome Liquors. 
acid solutions is increased by the 
addition of neutral chlorides and decreased by the addition of neutral 
sulfates. The effect of increasing concentration of different salts in 
solutions of sulfuric and hydrochloric acids was shown in Figs. 39 and 
40. In Fig. 129 are shown the changes in hydrogen-ion concentration 
occurring when various salts are added to solutions of the commercial 
chrome liquor and of pure chromic sulfate. The curves are strikingly 
similar to those in Figs. 39 and 40. The time effect is shown in the 
case of sodium chloride; the other measurements were made 30 days. 


30 20 10 10. 20 30 


acid alkali 


282 THE CHEMISTRY OF LEATHER MANUFACTURE 


after adding the salt in order to give time for the re-establishment of 
equilibrium. 

In order to avoid the complications obtained by mixing chlorides 
with chromic sulfate, Thomas and Baldwin also studied the effect of 
neutral chlorides upon a solution of pure chromic chloride. Fig. 130 
shows the effect of adding increasing amounts of various neutral salts 


Ammonium Sulfate 


(after 30 days) 
Sodium Sulfate 


Magnesium Sulfate 


PH Value of Chrome Liquor 


Moles Salt per Liter 


Fic. 129.—Effect of Neutral Salts on the Hydrogen-Ion Concentration of 
Chrome Liquors Containing 13.86 Grams of Chromic Oxide per Liter. 


to a solution of the green modification of chromium chloride; measure- 
ments of hydrogen-ion concentration were made immediately after 
adding the salt and diluting to definite concentration and also after the 
solutions had stood for 50 days. Where no salt was added, there was 
a rise in pH value upon standing. 

The complex nature of the time effect after adding salt to a chrome 
liquor is shown in Fig. 131. Commercial chrome liquor and sodium 
chloride were mixed and diluted so that the final concentration of 
sodium chloride was twice molar and of chromic oxide 13.86 grams per 
liter. Determinations of hydrogen-ion concentration were made every 


CHROME TANNING 283 


10 minutes for 4 hours and then at longer intervals for 3 days. It is 
evident that the action causing the increase in hydrogen-ion concentra- 


CHROMIC CHLORIDE SOLUTION 


pH Value of Chrome Liquor 


1 2 3 4 
Moles Salt per Liter 
Fic. 130.—Effect of Neutral Chlorides on the Hydrogen-Ion Concentration of a 


Solution of Chromic Chloride Containing 13.77 Grams of Chromic Oxide per 
parter, 


tion is subject to a time factor as well as the hydrolysis of the 
chromium salt. 

Following the observation by W. Klaber that chrome liquors can 
be made more basic, without causing precipitation, if salt be added to 


3,0 COMMERCIAL CHROME LIQUOR 
(13.86 grams Crj0, per liter) 


pH Value of Chrome Liquor 


4 8 ee 16 20 24 360 
Time in Hours 


Fig. 131.—Change in Hydrogen-Ion Concentration of Chrome Liquor with 
Time after the Addition of 2 Moles of Sodium Chloride per Liter. 


them previously Wilson and Kern® noted that the resistance of a 
chrome liquor to precipitation by alkali is increased by the addition of 


*The Action of Neutral Salts upon Chrome Liquors. J. A, Wilson and E, J. Kern 
J. Am, Leather Chem. Assoc. 12 (1917), 445. 


284 THE CHEMISTRY OF LEATHER MANUFACTURE 


all neutral salts‘and that sulfates are even more effective than chlorides. 
‘he amount of alkali required to start precipitation in a chrome liquor 
was determined by titrating 10 cubic centimeters of filtered liquor with 
o0.IN sodium hydroxide until the first permanent turbidity appeared, 
as seen by looking through the liquor. tor a given chrome liquor, 3.7 
cubic centimeters of standard alkali were required. To another portion 
of 10 cubic centimeters was added 0.04 gram molecule of sodium 
chloride; in this case 6.8 cubic centimeters of the standard alkali were 
required to start precipitation. Kepeating the experiment, taking in 
each case 10 cubic centimeters of the chrome liquor and 0.02 gram 
molecule of added salt, the following amounts of standard alkali were 
required to start precipitation in the presence of the neutral salt indi- 
cated: none, 3.7; KBr, 3.9; KCl, 40; KNO,, 4.2; NH Cl ais Nace 
5-4; MgCh, 0.2; MgsO,, 10.5; NazsO,, 11.4; and (NH,)25O,, 11.6. 
cit least some of the effect ot the chlorides may be attributed to their 
action in increasing the hydrogen-ion concentration of the liquor. ‘lhe 
sulfates, however, decrease the hydrogen-ion concentration, but, since 
they increase the stability of the chrome liquor, it seems likely that they 
form addition compounds with the chromium salt less easily precipi- 
tated than the simpler salt. 


Diffusion of Chromium Salts into Protein Jellies. 


In vegetable tanning, the rate of diffusion of tannin into the fibers 
of the skin increases with increasing pH value. In chrome tanning, 
the reverse is true. An increasing pti value causes the molecules of 
chromium salt to form aggregates ot increasing size, greatly reducing 
the rate at which they dittuse into the skin. 

When neutral skin substance is brought into contact with a chrome 
liquor, both the free acid present in the liquor and the basic chromic 
sait begin to dittuse into it. But the greater rate of diffusion of the 
acid causes the liquor to become more basic. Procter and Law +¥ 
studied the relative rates of dittusion of the free acid and chromium 
salt of a chrome liquor into gelatin jelly by allowing a faintly alkaline 
solution of gelatin and phenolphthaiein to set in a Nessler tube and 
pouring the chrome liquor on top of the jelly. As the acid diffuses 
into the jelly, it discharges the color of the phenolphthalein, while the 
extent ot diffusion of the chromium salt:can be followed by its color 
the combination of both acid and chromium with the gelatin has a 
retarding eftect upon the rate of diffusion. ) 

this differential diffusion may not occur where the common practice 
of pickling skins prior to tanning is used. When the skins contain a 
great excess of acid, the chromium salt diffuses into them very rapidly 
but the rate of combination of chromium and collagen is corresponding! 
agers and : becomes necessary to neutralize some of the acid Rees 
the skins can become completely tanned 
meated by the chromium cai » oven note ee ea 


70H. R. Procter and D. J. Law. J. Soc. Chem. Ind, 28 (1909), 297. 


CHROME TANNING 285 


The Time Factor in Chrome Tanning. 


The progress of the chrome tanning of hide powder with time has 
been studied by Thomas, Baldwin, and Kelly.11 12 They first examined 
the action of the commercial chrome liquor described above. The 
chrome liquor was diluted to contain 17 grams of chromic oxide per 
liter. Two hundred-cubic centimeter portions were poured upon 5-gram 
portions of hide powder in glass-stoppered bottles. These mixtures 
were kept at room temperature (about: 20-00: ); agitated frequently, and 
Gltered off at definite intervals,—1, 2, 4, 6, 8, 12, 24, 48, 72, and 96 hours. 
They were filtered by suction on a dry paper in a Buchner funnel, the 
Gltrate was set aside for analysis, and the tanned powder was washed 
with 500 cubic centimeters of water in order to remove chromium salts 


ro) 
° 
° 
° 
@ 


COMMERCIAL CHROME LIQUOR 


er Liter 
ed 
ro) 
ro) 
ro) 
J 


Pp 


™ 0.0005 


Moles Hydrogen-Ion 


10 20 30 40 50 60 70 80 90 
Time in Hours 


Fic. 132,—Change in Hydrogen-Ion Concentration of Chrome Liquors with 
Time. 


not chemically combined. The washed powder was partially dried at 
4o° C. and then completely at TOO, es 

The filtered liquors were analyzed for hydrogen ion, acidity, and 
chromic oxide and the tanned powders for sulfate, chromic oxide, ash, 
and hide substance (nitrogen x 5.62). 

Measurements of hydrogen-ion concentration were made immedi- 
ately upon filtration of the liquors and, in order to exclude the possibility 
of attributing natural hydrolytic changes to absorption by hide sub- 
stance, parallel measurements were made upon a portion of the chrome 
liquor having no contact with hide powder. The parallel sets of meas- 
urements are shown in Fig. 132. 

In Fig. 133 are shown the amounts of chromic oxide and of sulfate 
combined with 1 gram of skin protein at different intervals of time. 
These were obtained from the analyses of the washed powders after 
tanning. The broken lines represent calculations made from analyses 
of the chrome liquors on the assumption that the concentration is uni- 

11 The Time Factor in the Adsorption of the Constituents of Chrome Liquor by Hide 


Substance. A. W. Thomas, M. E. ‘Baldwin, and M. W. Kelly. J. Am, Leather Chem. 


Assoc. 15 (1920), 147. 
2 The Time Factor in the Adsorption of Chromic Sulfate by Hide Substance. A. W. 


Thomas and M. W. Kelly. Ibid., 15 (1920) 487. 


286 THE CHEMISTRY OF LEATHER MANUFACTURE 


form throughout all of the solution present in the system during tanning, 
We know from Chapter 5 that this assumption is not true, that the 
solution absorbed by the collagen jelly is less concentrated than the 
outer solution. This explains why these curves show lower values for 
combined sulfate and chromic oxide. Thomas and Kelly were well 
aware of this fact and presented the calculations made from the analysis 
of the solutions for the purpose of demonstrating the fallacies in this 


from analysis of leathers 


_from analysis of liquors 


Crg0z Combined with 
1 Gram Hide Substance 


”_— — 


Milligrams SOz Combined Megs, 
with 1 Gram Hide Substance 


10 20 30 40 50 60 70 80 90 
Time in Hours 
Fic. 133.—Progress of Combination of CrOs and SO; with Hide Substance 


during 4 Days’ Contact with Chrome Liquor containing 17 Grams Cr.O; per 
Liter. 


method, which is commonly used for measuring the extent of “adsorp- 
tion” of substances from solution by skin and other materials. The 
discrepancy is increased in the case of the sulfate determination by the 
fact that some combined sulfate is removed by washing, whereas the 
chromium-collagen compound is very stable. | 

They next .studied the action of a solution of pure chromic sulfate, 
but, using the same procedure, got only erratic results. They found 
it necessary, when using the pure salt, to soak the hide powder in water 
before adding the chrome liquor. The fact that the pure salt gave a 
solution very much more acid than the commercial salt may have had 
something to do with this. Five-gram portions of hide powder were 


CHROME TANNING 287 


5 
ha 
0,020 
F<} 
Ma 
mw ® 0,015 
| 
10,0104 ~~ 
ae 
@ 0,005 
a 
Z = 
10 20 30 40 50 60 70 64 
Time in Hours days 


Fic. 134.—Change in Hydrogen-Ion Concentration of Chrome Liquors with 
Time. 


placed in each of a series of glass-stoppered bottles, 50 cubic centimeters 
of water were added to each, and the powders were allowed to soak over 
night. Then 150 cubic centimeters of chromic sulfate solution were 


r= 
+ 2 150 
La 
3+ 125 Seah 
Ree eR ee — 
a3 oad 
ve 

421007) // CHROMIC SULFATE SOLUTION 
83 751 
wen «75 from analysis of leathers 
eaey OO 
eid. 2° from analysis of liquors _ 
= 

@ 140 
32 
8&8 120 
a CHROMIC SULFATE SOLUTION 
£0 Pa ee Se teres SE 
aes 100 2 Wee es cate Aine wail, 

/ 

ere 80 / 
ff at / from analysis of leathers 
2) { Bihar em, ipeee 
d 8 from analysis of liquors 
Mo) ae emeetn epee Te aa Ae ph ey bald ee eee 
oe 
ee 20 
23 

Ca! 

z 


5 10 15 20 25 30 35 64 
Time in Days 
Fic. 135.—Progress of Combination of Cr.O; and SO; with Hide Substance 


during 64 Days’ Contact with Chromic Sulfate Solution containing 16.4 
Grams Cr,Os per Liter. ; 


288 THE CHEMISTRY OF LEATHER MANUFACTURE 


added, making 200 cubic centimeters of liquor with a concentration of 
16.4 grams of chromic oxide per liter. The rest of the experiment was 
performed just as in the case of the commercial chrome liquor, except 
for the fact that a time period of 64 days was covered. 

The variation in hydrogen-ion concentration in the control liquor 
and in the filtrates from the tanning tests is shown in Fig. 134. The 
trend of the curves is different from that of those in Fig. 132. 

The amounts of chromic oxide and of sulfate combined with 1 gram 
of hide substance are shown in Fig. 135. When the calculations were. 
made from the analyses of both leathers and liquors, the same sort of 
differences were noted as with the commercial chrome liquor. The 
only reliable figures are those obtained from the analyses of the leathers, 
shown by the continuous lines. The amount of chromic oxide combined 
with 100 grams of hide substance approaches a limiting value of 13.8 
grams. Because this is almost exactly 4 times the author’s value of 
3.38 grams, calculated to be the smallest amount of chromic oxide 
required to convert 100 grams of collagen into the chromium salt, 
Thomas and Kelly referred to it as tetrachrome leather. 

A comparison of Figs. 133 and 135 will show that the rate of tanning 
is very much less in the chromic sulfate solution, in which the hydrogen- 
ion concentration is about 20 times as great as in the commercial liquor. 
It will also be noted that the amount of chromic oxide combined with 
I gram of skin protein is greater for the commercial liquor after 4 days 
than the limiting value in the case of the pure chromic sulfate and has 
not reached a limiting value in 4 days. The significance of this will 
be made more apparent presently. 


The Concentration Factor in Chrome Tanning. 


The effect of the concentration of a chrome liquor upon the fixation 
of chromium by skin protein has been studied by Baldwin ** and by 
Thomas and Kelly.***° A solution of the commercial chrome liquor, 
described above, was made having a concentration of 202 grams of 
chromic oxide per liter and this was used at various dilutions to study 
the effect of concentration. A 200-cubic centimeter portion of each 
dilution was poured into a bottle containing hide powder equivalent to 
5 grams of water-free hide powder. Another portion of each solution 
was set aside and at the expiration of 48 hours the hydrogen-ion con- 
centration was determined. The bottles were shaken at intervals for 
48 hours and the contents were then filtered off by suction. Analyses 
were made of the liquors and of the tanned powders after washing and 
drying, the methods employed being the same as in the studies of the 
time factor. , 

% The Effect of the Concentration of a Chrome Liquor upon Adsorption by Hide Sub- 
stance. M. E. Baldwin. J. Am. Leather Chem. Assoc. 14 (1919), 433. 

%* The Effect of Concentration of Chrome Liquor upon the Adsorption of Its Constitu- 


ents by Hide Substance. A. W. Thomas and M. W. Kelly. J. Ind. Eng. Chem. 13 


(1921), 31. 
* Equilibria between Tetrachrome Collagen and Chrome Liquors; the Formation of 
Octachrome Collagen. Ibid., 14 (1922), 621. 


CHROME TANNING 289 


Fig. 136 shows the variation in hydrogen-ion concentration in the 
filtrates and also in the control liquors which had no contact with hide 
powder. Fig. 137 shows the effect of concentration upon the amount 
of chromic oxide fixed by 1 gram of skin protein in 48 hours. Where 
the calculation was made from the analyses of the liquors, a ridiculous 
result was obtained, as expected. In this calculation it is assumed that 
the decrease in concentration of the liquor represents the amount of 
solute combined with the hide powder, but the concentration of the 
stronger liquors is actually increased by the introduction of hide powder, 


0,006 


(COMMERCIAL CHROME LIQUOR 


0,005 


0,004 


0,003 


Moles Hydrogen Ion per Liter 


25 50 Ot we eLOO. sel 25 
Grams Chromic Oxide per Liter 


Fic. 136.—Change in Hydrogen-Ion Concentration of Chrome Liquors with 
Increasing Concentration. 


due to the absorption of a greater proportion of water to chromium 
salt than existed in the solution before the introduction of the hide 
powder. It is interesting to note that the method of calculation giving 
these ridiculous results is the same in principle as that of the official 
method of tannin analysis of the American Leather Chemists Associa- 
tion, described in Chapter 12. The determination of combined chro- 
mium by analysis of the washed leathers corresponds to the Wilson-Kern 
method of tannin analysis, also described in Chapter 12. 

The reason for the point of maximum at a concentration of 15 
grams of chromic oxide per liter is not entirely clear, although a number 
of causes may be assigned to the falling off in rate of combination at 
higher concentrations, among which may be mentioned the increasing 
hydrogen-ion concentration, as shown in Fig. 136, the increasing salt 
concentration, and the probability of the formation of addition com-: 
pounds, It is interesting to compare this curve with those for the rate 


2900 THE CHEMISTRY OF LEATHER MANUFACIORS 


of vegetable tanning as a function of concentration, shown in Chap- 
ter 13. 

The experiments just described were repeated exactly, except for 
the fact that the hide powders were kept in the chrome liquors for 8.5 
months. The results are shown in Fig. 138. Curiously enough a point 
of maximum occurs having a value of 26.6 grams of chromic oxide per 
100 grams of skin protein, which is approximately 8 times the author’s 
calculated minimum of 3.38. Moreover a point of inflection occurs in 
the curve at a point giving just half of the maximum value. On the 


(from analysis 
of leathers) 


LIQUOR 


Milligrams Cro0q Combined 
with 1 Gram Hide Substance 


50 100 150 200 
Grams Chromic Oxide per Liter 


Fic. 137.—Effect of Concentration of Chrome Liquor upon Combination of 
Chromium with Hide Substance. 


assumption that they had obtained octachrome collagen, Thomas and 
Kelly calculated the combining weight of collagen as 94, a value of the 
same order of magnitude as those of the amino acids making up the 
protein molecule. This degree of combination of collagen and chro- 
mium is the highest ever reported in the literature. 

The fact that only a tetrachrome collagen was obtained after 64 
days of contact with chromic sulfate solution, whereas an octachrome 
collagen was obtained with the commercial chrome liquor may possibly 
be explained by the differences in hydrogen-ion concentration of the 
two series of liquors. On this assumption, only half of the total number 
of carboxyl groups are capable of attaching chromium bonds at the 
higher acidities. The possibility is thus suggested that a series of col- 
lagen salts from monochrome to octachrome might be obtained by 
tanning hide powders for a sufficient length of time at different pH 


CHROME TANNING 291 


values. Experiments to settle this point will be made as soon as the 

opportunity affords. 
The reversibility of the action leading to the formation of tetra- 

chrome collagen was studied by Thomas and Kelly. Since the chrome 


Tetrachrome 
Collagen 


"4 


Milligrams Cro0, 
or 50g Combined with 
1 Gram Hide Substance 


50 100 150 200 
Grams Chromic Oxide per Liter 


Fic. 138.—Effect of Concentration of Chrome Liquor upon Combination of 
Chromium and Sulfate with Hide Substance. 


tanned hide powder does not lose any measurable amount of chromium 
upon several hours’ washing, they decided to allow tetrachrome collagen 
to remain in contact with solutions of varying chromium content for 
several months. 


(after 8.5 months) 
Cro0g 


Milligrams Cro0g 
or 50g Combined with 
1 Gram Hide Substance 


50 100 150 200 


Grams Chromic Oxide per Liter 


Fic. 139.—Effect of Concentration of Chrome Liquor upon Combination of 
.Chromium and Sulfate with Tetrachrome Collagen. 


Portions of tetrachrome collagen containing just 5 grams of hide 
substance were placed in a series of 12 bottles and covered with 200- 
cubic centimeter portions of chrome liquor of various concentrations. 
The bottles were kept sealed to prevent evaporation and were shaken 
once a week. At the end of 8.5 months the contents were filtered and 


292 THE CHEMISTRY OF LEATHER MANUFACTURE 


< 


the powders washed free from soluble matter, dried, and analyzed. 
The results are shown in Fig. 139. They show that a hydrolysis of 
the chrome collagen and collagen sulfate compounds takes place in 
water and in very dilute chrome liquor, which was also shown by an 
increase in hydrogen-ion concentration. 

With further increase in concentration of the chrome liquor, there 
is a steady addition of Cr,O; and SOs, approaching the condition of 
octachrome collagen. But with further increase in concentration the 


COMMERCIAL CHROME LIQUOR 


@ 
oO 


(17 grams Crp0, per liter) 


on oO ~ 
oO oO oO 


Hide Substance in 24 Hours 
nw nes 
oO oO 


(pickled calf skin) 


Milligrams Crp0z Combined with 1 Gram 


1 2 3 
Moles of Added Salt per Liter 


Fic. 140.—Retardation of Chrome Tanning by Neutral Salts at Various 
Concentrations. 


curve does not fall below the tetrachrome value. This shows that the 
curve in Fig. 138 does not represent the equilibrium condition of a 
reversible reaction. On the contrary, it suggests that the fall in the 
curves is due to the increasing hydrogen-ion concentration, which inhibits 
the combination of collagen and chromium. 


Effect of Neutral Salts upon Chrome Tanning. 


In studying the effect of neutral salts upon the chrome tanning of 
calf skin, Wilson and Gallun 1° selected sodium sulfate and the chlorides 
of ammonium, sodium, lithium, and magnesium because of their differ- 
ent degrees of hydration in aqueous solution. The commercial chrome 


7° The Retardation of Chrome Tanning by Neutral Salts. J. A. Wilson and E. A. 
Gallun. J. Am. Leather Chem, Assocz 15 "C1g20), 273. ; 


CHROME TANNING 203 


Moles of Salt per Liter 


CREB EAU Ess 


{Tf p) 


Kind of Salt 


Lithium Chloride \) / | 
Magnesium Chloride ( 


Fig. 141.—Strips of Chrome Leather, All Originally of Equal Area, 
Taken After Tanning for 24 Hours and Then Boiling in Water for 
5 Minutes and Drying. Compare with Curves in Fig. 140. 


294 THE CHEMISTRY OF LEATHER MANUFACTURE 


mixture which they used showed by analysis: Cr2Os3, 24.2 per cent; 
Fe,Os;, 0.6 per cent; Al,Os, 2.7 per cent; SOs, 39.5 per cent; Cl, 0.4 
per cent; and basicity corresponding to the formula Cr(OH)1.4(SO.)o.s. 
Solutions of this chromium preparation were mixed with solutions of 
the various neutral salts so as to give liquors containing 17 grams of 
chromic oxide per liter and definite quantities of salt. 

In each test a strip of pickled calf skin, 16 square inches in area, 
was covered with 200 cubic centimeters of chrome liquor, in a bottle, 
and shaken at intervals for 24 hours. The pieces of skin were then 
washed by shaking with successive changes of water until the wash 
water gave only a very faint test for chloride or sulfate. Strips of 
equal area were cut from each piece and immersed in boiling water 
for 5 minutes, in order to determine the nearness to complete tannage. 
The remaining portions were cut into small pieces, dried and analyzed. 
The effect of increasing concentration of the chlorides of ammonium, 
sodium, lithium, and magnesium upon the amount of chromic oxide 
combined with a unit of hide substance in 24 hours is shown in Fig. 
140. In Fig. 141 are shown the strips of skins after being subjected to 
the action of boiling water for 5 minutes. The appearance of these 
strips parallels the curves, in a rough sort of way; that is, they gen- 
erally show the greatest shrinkage in area where the amount of com- 
bined chromic oxide is smallest. The first strip in each series proved 
to be fully tanned and showed no shrinkage in area. 

In another series of tests, chrome liquor having a concentration 
of 10 grams of chromic oxide per liter was used and the effect of 
sodium sulfate was studied in addition to the effect of the chlorides 
noted above. The results for the chlorides were practically the same 
as in the first experiment, except for the observation of a point of 
minimum in the sodium chloride curve at 2 moles per liter of salt and 
a slight rise to 3 moles. In the case of the sodium sulfate, there was 
a steady drop in fixation of chromic oxide from 10.09 grams per 100 
of hide substance when no added salt was present to 3.57 when the 
solution was saturated with the salt. 

Wilson and Gallun attributed the action of the chlorides, in part, 
to their hydration. The removal of .water from the réle of solvent 
would have the practical effect of increasing the concentration of all 
of the constituents of the chrome liquor, expressed in terms of the 
free solvent. Fig. 137 shows that increasing the concentration of the 
chrome liquor from 17 grams of chromic oxide per liter does retard 
the rate of combination of collagen and chromium. That the chlorides 
actually concentrate the solute in the free solvent is also indicated by 
the fact that they increase the hydrogen-ion concentration as measured 
by the hydrogen electrode. But at half-molar concentration the effect. 
only of magnesium chloride is in its expected relative position; being 
most highly hydrated it should produce the greatest effect, which it does. 
At 3-molar concentration all 4 chlorides are in an order inverse to that 
expected unless it may be assumed that at very high concentrations of 
chrome liquor a further increase in concentration causes an increased 
rate of tanning. 


CHROME TANNING 295 


The action of sulfates is in a category different from that of the 
chlorides, as shown by the fact that they decrease the hydrogen-ion 
concentration of acid solutions and of chrome liquors. Wilson and 
Gallun suggested that the retarding action of sodium sulfate was prob- 
ably due to the formation of addition compounds between the added 
salt and the chromium compounds which tan less readily than the 
original chromium compounds, but that the action was further com- 
plicated by the hydration of the sodium sulfate. 


a 
H 
5 
m 120 
@ 
s 
g 110 
16. ° 

§.5 28 Cr,0, per liter aaerone 
S 1004 

ws 1 a 
< AGS See A Se aeceaaaed 

DOA’ Naso 
A oe Dae eee Nall 
© a ‘\ Bots oneers 
e 80 ay ne vent eer as 
is] SS ces 2 Nees. Sp ea, 
q Ate Nacl 
Bt, 70 
i 
=) 60 
» 
: Na,s 
a,90 
4 <N Se 
8 0 pee tine 
Se) 3 
N 
°. 30 cae 
b ra 
= Saas ape OA 
: : 
pe Cad Ber ee 
4 
ot 
a 
1 ic 3 4 


Moles Added Substance per Liter 


Fra. 142.—Retardation of Chrome Tanning by Neutral Salts and by Sucrose at 
Various Concentrations. 


In the hope of throwing further light on this complex action, Thomas 
and Foster 2“ studied the actions of sodium chloride, sodium sulfate, 
and sucrose upon chrome tanning. They prepared a pure chrome liquor 
by reducing pure sodium bichromate with sulfur dioxide and then 
expelling the excess of this gas. Portions of hide powder equal to 
5 grams of hide substance were covered with 50 cubic centimeters of 


Influence of Sodium Chloride, Sodium Sulfate and Sucrose on the Combination of 
Chromic Ion with Hide Substance. A. W. Thomas and S. B. Foster. J. Ind. Eng. Chem. 


14 (1922), 132. 


296 THE CHEMISTRY OF LEATHER MANUFACTURE 


water in bottles and allowed to stand over night, when the salt or sugar 
to give the desired concentration was added. Finally 150 cubic centi- 
meters of chrome liquor were added, of such concentration that if it 
were diluted to 200 cubic centimeters it would contain 3, 15.5, or 100 
grams of chromic oxide per liter. The mixtures were rotated in a 
tumbling machine for 48 hours, filtered through muslin bags, and 
washed well with tap water and 3 times with 200-cubic centimeter 
portions of distilled water. The washed powders were then dried and 
analyzed as usual. 

The effect of increasing concentration of sodium sulfate, sodium 
chloride, and of sucrose is shown in Fig. 142. Analysis of the filtrates 
from the series of chrome liquors of intermediate concentration showed 
that increasing concentration of sodium chloride lowered the pH value 
from 2.90 to 2.20, while sodium sulfate raised it to 3.01. This con- 
trasting effect is similar to that shown in Fig. 120. | 

The curves representing the effect of sodium chloride all have points 
of minimum followed by an upward trend, which, in the case of sodium 
sulfate, is shown only where the chrome liquor is very concentrated. 
But sucrose is evidently without effect, except at 4-molar concen- 
tration. 

Because sucrose, which is hydrated in aqueous solution, shows no 
retarding effect upon chrome tanning up to 3-molar concentration, 
Thomas and Foster concluded that the retarding effect of the salts is 
probably due to some cause other than their hydration. They suggested 
that chlorides, as well as sulfates, form addition compounds with 
chromium salts rendering them less dissociated and, consequently, less 
active in combining with the skin protein. They liken the action to the 
decrease in toxicity of mercuric chloride in the presence of sodium 
chloride, which Rona and Michaelis '* ascribe to the formation of the 
complex ions HgCl,;’*and HgCl,’”.. Upon increasing the concentration 
of salt still further, the hydration effects a virtual concentration of the 
chromium ions to such an extent that the retarding action of the 
addition compound formation is counterbalanced by the activity of 
the high concentration of chromium ion and the curves begin to slope 
upward. 

It may be questioned as to whether we are safe in drawing con- 
clusions regarding the effect of hydration from the experiments with 
sucrose. Corran and Lewis *® found that potassium and chloride ions 
are soluble in the water of hydration of sucrose. The effect of neutral 
salts upon the hydrogen-ion activities of acid solutions, described in 
Chapter 4, seem to indicate that ions are not soluble in the water of 
hydration of salts. Thomas and Foster ascribe the retardation of 
chrome tanning by 4-molar sucrose to the formation of a compound 
with chromium, analogous to the combination with other hydroxy 
compounds, such as tartrates. As will be shown presently, such com- 
pounds of chromium have no tanning power. : 


8 Biochem. Z. 97 (1919), 85. 
% The Effect of Sucrose on the Activities of the Chloride and Hydrogen Ions. Jew 
Corran and W. C. M. Lewis. J. Am. Chem. Soc. 44 (1922), 1673. 


CHROME TANNING 207 


Effect of Salts of Hydroxy-Acids upon Chrome Tanning. 


While investigating the failure of certain types of chrome liquor 
to tan pickled calf skin, Procter and Wilson *° found that the tanning 
action is checked and may even be reversed by the introduction of 
salts of hydroxy-acids into the chrome liquor. 

After the addition of Rochelle salt (potassium sodium tartrate) 
to a chrome liquor, a change occurs in the liquor which requires a 
considerable length of time for completion. Immediately after the 
addition, the chrome liquor is still precipitable by alkali, but the pre- 
cipitate slowly redissolves with time. If the liquor is allowed to stand 
for several hours before the addition of alkali, no precipitate forms 
at all. A color change in the liquor is also noticeable. 

A chrome liquor was made by rendering a solution of chrome alum 
basic with sodium carbonate and from this a series of solutions was 
prepared having a concentration of 13 grams of chromic oxide per 
liter and Rochelle salt in concentration ranging from zero to 50 grams 
per liter. A piece of calf skin was put into each solution, which was 
agitated occasionally for 24 hours. At the end of this time, all pieces 
in solutions containing 10 grams per liter or less of Rochelle salt were 
completely tanned, as determined by the boiling test, but those in the 
stronger solutions were not. ‘These solutions all gave precipitates upon 
addition of sodium carbonate; that containing 25 grams of the salt per 
liter gave a slight precipitate and that containing 50 grams gave none. 
The piece in the solution containing 25 grams became tanned after the 
addition of sodium carbonate but that in the solution containing 50 
grams could not be made to tan, regardless of the amount of alkali 
added. That the action of the Rochelle salt was on the chromium salts 
and not on the skin was proved by washing the skin that was still not 
tanned and placing it in a liquor containing no Rochelle salt, in which 
it quickly became fully tanned. 

The same results were obtained when the experiment was repeated 
with sodium citrate. The sodium salts of lactic, gallic, and salicylic 
acids were also found to prevent the precipitation of chrome liquor 
by alkali. Procter and Wilson attributed the action of Rochelle salt 
to the formation of a complex chromi-tartrate ion analogous to the 
cupri-tartrate ion of Fehling’s solution. 

The fact that Rochelle salt will dissolve precipitates of chromic 
hydroxide led Procter and Wilson to suspect that it might have the 
power to decompose chrome leather. They soaked a piece of fully 
tanned leather in a normal solution of Rochelle salt over night and 
found, next day, that it would not stand the boiling test. But after 
washing and soaking in fresh chrome liquor, containing no Rochelle 
salt, it soon became fully tanned again. It was found that chrome 
leather can be detannized by Rochelle salt solution and then tanned 
again in fresh chrome liquor repeatedly, showing that chrome tanning 
is a reversible action, under certain conditions. 


20The Action of Salts of Hydroxy-Acids upon Chrome Tanning. H. R. Procter and 
J. A. Wilson. J. Soc. Chem. Ind. 35 (1916), 156. 


Fig. 143.—Vertical Section of Calf Leather. 
(Vegetable tanned.) 


Location: butt. Eyepiece: none. 

Thickness of section: 40 p. Objective: 16-mm. 

Stain: none. Wratten filter: K3-yellow. 
Tannage: vegetable. Magnification: 80 diameters. 


298 


Fig. 144.—Vertical Section of Calf Leather. 
(Chrome tanned.) 


Location: butt. Eyepiece: none. 

Thickness of section: 40 nv. Objective: 16-mm. 

Stain: none. Wratten filter: K3-yellow. 
Tannage: chrome. Magnification: 80 diameters. 


299 


300 THE CHEMISTRY OF LEATHER MANUFACTURE 


The extent to which chrome leather can be freed from chromium 
by Rochelle salt was shown by soaking a piece of chrome leather in 
a normal solution of Rochelle salt for two weeks. The solution was 
colored a deep green and the skin, after thorough washing, was found 
to be practically free from chromium and resembled a piece of bated 
skin. Upon heating with pure water, it was gradually converted into 
gelatin and the solution set to a firm jelly upon cooling. This work has 
since found application in preparing chrome leather wastes for manu- 
facture into glue and jor the stripping of the chrome from the surface 
of leather to be retanned with vegetable tanning materials. 


Comparison of Chrome and Vegetable Tanned Leathers. 


Ever since chrome tanning was first introduced, the relative merits 
of chrome and vegetable tanned leathers have formed the subject- 
matter for debate. Too often, however, the attempt was made to com- 
pare a poor grade of one kind of leather with a good grade of the other, 
without taking into consideration differences in the original skins and 
in the methods of manufacture of the leathers. Since the resistance 
of a leather to tearing, for example, is a function of the grease content, 
the moisture content, and the extent to which the thickness of the 
original skin has been reduced by splitting, any comparison between 
two kinds of leather must take all of these factors into consideration. 
There are, however, certain differences between chrome and vegetable 
tanned leathers that are incontrovertible and more or less independent 
of the details of manufacture. These only will be considered in mak- 
ing the comparison. | rie 

Figs. 143 and 144 represent vertical sections of vegetable and chrome 
tanned leathers made from the same skin. After bating, the skin was 
cut into halves along the line of the back bone. One half was tanned 
with chrome liquor and the other with vegetable tanning materials. 
When finished, each leather represented an excellent specimen of shoe 
upper leather of its particular kind. The sections shown in the figures 
are from the finished leathers and were cut from exactly corresponding 
points on the skin. a 

The outstanding difference in appearance is the much larger size 
of the fibers of the vegetable tanned leather. In the chrome tanned 
leather, the fibers are thin, as in dried, raw skin, but in the veg- 
etable tanned leather, the fibers have grown to such an extent that 
they almost completely fill the interfibrillary spaces. But this differ- 
ence in size of the fibers is only what one would expect from the fact 
that 100 grams of skin protein combined with 57.0 grams of tannin, 
in the case of the vegetable tanned leather, as. against only 7.2 grams 
of chromic oxide, in the other. This difference is responsible for 
the greater weight and solidity of vegetable tanned leather. Either 
leather can be made tough and as soft as desired by the introduction 
of a sufficient amount of oil, but the vegetable leather is capable of 


Fig. 145.—Vertical Section of Horse Leather. 
(Cordovan—from shell.) 


Loeation: butt. Eyepiece: none. 

Thickness of section: 20 nu, Objective: 16-mm. 

Stain: none. Wratten filter: K3-yellow. 
Tannage: chrome, Magnification: 70 diameters, 


301 


Fig. 146.—Vertical Section of Goat Leather, 


Location: butt. 

Thickness of section: 40 uw. 
Stain: none. 

Tannage: chrome. 


Eyepiece: 5X. 

Objective: 16-mm. 

Wratten filter: H-blue green. 
Magnification: 92 diameters. 


302 


Fig. 147.—Vertical Section of Slink Calf Leather. 
(Black ooze calf.) 


Location: butt. Eyepiece: 5X. 

Thickness of section: 30 UL. Objective: 16-mm. 

Stain: none. Wratten filter: B-green. 
Tannage: chrome. Magnification: 105 diameters, 


393 


304 THE CHEMISTRY OF LEATHER MANUFACIURe 


absorbing a much greater quantity of oil without becoming raggy. By 
comparison with the vegetable leather, the chrome leather feels 
empty. : 

Another outstanding difference between the two kinds of leather 
is the relatively high sulfuric acid content of the chrome tanned leather 
compared to the negligible amount present in the vegetable leather. 
This particular sample of leather showed by analysis 6.65 grams of 
sulfuric acid per 100 grams of skin protein, which is typical of leathers 
of this type on the market. It should be recognized, however, that 
this sulfuric acid is not entirely free, but is combined either with 
the chromium compounds or with the skin protein. Only a trace 
of acid is free at any one time, but as soon as this trace is removed by 
washing with water, more is immediately liberated by hydrolysis. 

Attempts to free chrome leather from sulfuric or other mineral 
acid, without damaging the leather in some way, have not been success- 
ful, so far as the author is aware. Reducing the content of sulfuric 
acid much below that normally occurring in chrome leather seems to 
cause brittleness, although the cause of this is not known. In making 
comparative tests, the author has always found shoes made from veg- 
etable much more comfortable, especially on long walks, than shoes 
from chrome leather and has attributed at least part of this difference 
to the hydrolyzable sulfate present in the chrome leather. 

The rise of chrome tanning has been favored by its speed and 
comparative simplicity. The manufacture of vegetable leathers requires 
a much longer time and more labor. In the manufacture of light 
leathers, not sold by weight, tanners have naturally preferred to 
switch to the quicker method of tanning with chromium salts, although 
some of the best grades of upper leather are still tanned with vegetable 
‘tanning materials. In the manufacture of heavy leathers, sold by 
weight, tanners have been forced to adhere to the older method of 
vegetable tanning in order to get profitable yields. Incidentally, the 
author believes that they also get better leather. | 

In Fig. 145 is shown a vertical section of chromed tanned horse 
hide, which should be compared with Fig. 105, of Chapter 13. Both 
of these sections are from corresponding points of the same hide, 
which was cut in two after bating, half being tanned in chrome liquor 
and half in vegetable tan liquors, as in the experiment with calf skin. 
The difference between the two kinds of tannage is even more noticeable 
here. 

Fig. 146 shows a fine specimen of chrome tanned goat skin, such 
as is used in making kid shoes. Fig. 147 represents an unusually 
fine specimen of ooze, or suede, leather. This leather is worn with 
the flesh side out, which gives it a velvety appearance. For the best 
grades, only slink skins are used because these are usually free from 
the blood vessels which are ordinarily abundant at the flesh boundary 
of the skin and detract from the appearance of leather finished on 
the flesh side. In comparing the various sections, any differences in 
magnification noted must be taken into consideration. 


CHROME TANNING 305 


Theory of Chrome Tanning. 


The simplest theory of chrome tanning is that it consists of the 
combination of chromium and collagen, forming a series of salts that 
might be called collagenates of chromium. From this chemical theory, 
there are theories of many shades and kinds all the way down to the 
assumption that chrome tanning consists of a precipitation of colloidal 
chromic oxide upon the surfaces of the skin fibers. 

In following chrome tanning by means of the microscope, the 
author has observed the chrome liquor diffuse into the skin and also 
into the substance of each fiber, but without any visible sign of 
precipitation. When tanning was complete, each fiber looked like a 
transparent rod of green glass. This is similar to the phenomenon 
observed in vegetable tanning. 

Thompson and Atkin ** recently attempted to apply the Procter- 
Wilson theory of vegetable tanning to chrome tanning. The electrical 
charge on the collagen may be accepted as positive during chrome 
tanning and it seemed improbable to Thompson and Atkin that com- 
bination takes place between positively charged collagen and a positively 
charged chromium ion or complex. In a review of 80 papers dealing 
with chromium salts, they found numerous contradictions on points 
of importance, but one fact seemed to stand out as definitely estab- 
lished. Chromic sulfate exists in solution in two modifications, one 
green, the other violet.. At any temperature between 0° and 100° C., 
a definite equilibrium exists between the two forms in solution, the 
green being more stable at high and the violet modification at lower 
temperatures. In the change from violet to green by raising the tem- 
perature, equilibrium is quickly reached, but in the reverse action, fol- 
lowing a lowering of the temperature, equilibrium is reached only after 
a long time. Thompson and Atkin offered the theory that chrome 
tanning is effected by an anion or negatively charged colloidal particle 
containing chromium and arising from the green modification of chromic 
sulfate. The action would then be similar to that described in the 
Procter-Wilson theory of vegetable tanning. 

In support of their theory, Thompson and Atkin cite a number of 
investigations showing that anodic migration occurred in the electro- 
phoresis of solutions of the green modification of chromic sulfate. 
It was pointed out by Seymour-Jones,?? however, that this theory would 
be acceptable only provided it could be shown that all chrome liquors 
capable of tanning contain this negatively charged chromium complex. 

Bassett ** electrolyzed solutions obtained by the reduction of 
potassium bichromate with sulfur dioxide. With fresh, dilute solu- 
tions, no precipitate was obtained with either barium chloride or 
ammonium hydroxide, indicating the absence of sulfate or chromic 


*1 A Possible Theory of Chrome Tanning. F. C. Thompson and W. R. Atkin. J. See 
Leather Trades Chem. 6 (1922). 207. 

a ey pee aos nomesis of Chromic Solutions. F. L. Seymour-Jones. Ind. Eng. Chem, 
15 (1923), 265. 

J. Chem, Soc. 83 (1903), 692, 


306 THE CHEMISTRY OF LEATHER MANUFACTURE 


ions. Bassett assumed the presence of complex chromo-sulfates of the 


type Seon: When fresh green solutions, containing a 


slight excess of sulfur dioxide, were electrolyzed under dilute sulfuric 
acid, a green boundary moved to the anode and a violet boundary to 
the cathode. With lapse of time the anodic migration decreased in 
speed and finally ceased altogether. 

Seymour-Jones points out that the cessation of anodic migration on 
standing is important to the theory, since it implies the breaking 
up of the negative complex to potassium sulfate, potassium sulfite, 
and chromic sulfate, ‘and, consequently, the absence of chromium in a 
negative complex in ordinary chrome liquors. Moreover, if the green 
solution is due to chromic anion and the violet to chromic cation, 
pure violet solutions should not tan, according to the Thompson- 
Atkin theory, but Burton 24 found that the violet modification tans more 
rapidly than the green. This seemed a natural finding in view of 
the fact that Blockey 2° had previously shown that the hydrogen-ion con- 
centration of solutions of the green modification is very much higher 
than that of solutions of the violet modification. 

Ricevuto 2* electrolyzed a 10-per cent solution of chrome alum and 
observed migration only to the cathode. Upon addition of sodium 
hydroxide, the solution became turbid and some particles moved to 
the anode. When the solution was rendered alkaline, the particles 
moved entirely to the anode, from which Ricevuto concluded that 
chrome tanning is possible only in alkaline solution, which we know 
. to be quite contrary to fact. 

Seymour-Jones carried out a number of electrophoresis experi- 
ments on various chromic solutions for the purpose of testing the 
Thompson-Atkin theory. A U-tube was used which was provided 
with a stopcock in each arm a little above the bend. The chromic 
solution was placed in the bend below and up to the top of the stop- 
cock. Above this was placed a 0.05-molar solution of sodium sulfate. 
The U-tube was connected by stoppers with electrodes in small dis- 
tilling flasks, copper in saturated copper sulfate serving as the cathode 
and platinum in saturated sodium chloride solution as the anode. 
Diffusion of these solutions was prevented by cotton wool plugs. The 
ordinary house current of 110 volts D. C. was used, passing through © 
a lamp filament to reduce the amperage to a convenient amount. The 
results are given in Table XX XVII. 

Wherever chrome liquor moved to the anode, it was of a pure 
green color, while that moving to the cathode was of a bluish green. 
The basic chromic chloride, which showed no anodic migration at 
all, was used to tan hide powder, which it did in quite the normal 
manner. This proves that the Thompson-Atkin theory is not of gen- 
eral application and suggests that it may not hold in any case. 


24 Chrome Tanning, I. D. Burton. J. Soc. Leather Trades Chem. 4 (1920), 205. 

25 Investigation of One Bath Chrome Liquors. J. R. Blockey. J. Soc. Leather Trades 
Chem, 2 (1918), 205. — : = , 2 1g SN od Bes 
Rai) 88 Kolloid Z. 3 (1908), 114. 


CHROME TANNING 207 


Seymour-Jones 77 experimented upon the ultrafiltration of basic 
chromic sulfate, such as is used in tanning, and found no colloidal 
matter. A chrome liquor was prepared by reduction of a solution of 
sodium bichromate with sulfur dioxide, the excess sulfur dioxide being 
driven off by boiling. The dark green solution contained 269.9 grams 
of chromic oxide per liter. This was ultrafiltered through hard filter 
papers impregnated with 1-per cent and 5-per cent gelatin solutions, 
respectively, which were subsequently hardened with 4-per cent for- 
maldehyde solution. The solution was also ultrafiltered through a 


ks Let AON KO Le 
DIRECTION OF MIGRATION OF CHROMIUM UPON ELECTROLYSIS OF CHROME Liquors. 


Solution Concentration Migration to 


NazCr.O; reduced with SO.. Solution 
13 months old. All free SO2 re- 239.9 grams of Anode and Cath- 


MMT adic ay 6 633s oe 014 6 0s Cr.O; per liter ode 
Cr2(SO,)s. Fresh, cold, green solu- 

Se Oo ee 0.2-molar Cathode only 
Cr2(SO.):. Heated and cooled green 

es eee er 0.2-molar Cathode only 
Cr2(SO.)s plus NaOH to make 

OU Sek ee 0.2-molar Anode and Cath- 
Commercial chrome crystals, basic ode 

eet c ek bos o's cea sepes 166.5 grams of Anode and Cath- 

Cr2O; per liter ode 

CrCl, plus NaOH to make CrOHChk. 45 Bans CrCl; Cathode only 
per liter 
Chrome alum. Fresh, cold violet 

NER I ie a ie os vs oo coe 0.2-molar Cathode only 
Chrome alum. Heated and cooled, 

PGMS OIUIOI ec cc dacs se ccsccs es 0.2-molar Cathode only 
Chrome alum. Heated and cooled, : 

plus NaOH to make CrOHSO:... —0.2-molar Cathode only 


collodion disc. In every case the solution passed through unchanged, 
no colloidal particles being retained by the filter. The same result 
was obtained when the solution, diluted with 3 volumes of water, was 
allowed to remain in a collodion bag suspended in air. The original, 
concentrated solution and one diluted to 10 volumes with water were 
dialyzed in collodion bags against water, which was changed frequently. 
In less than 18 hours even the concentrated solution had completely 
dialyzed through the membrane, the liquid remaining in the bag being 
colorless. This shows that we are not dealing with a colloidal dis- 
persion in chrome tanning. 

T. W. Richards and F. Bonnet 7° electrolyzed a basic chromic sulfate 
solution and found the migration entirely cathodic and there were 19.3 
grams of Cr transported per 96,580 coulombs. From this they con- 
cluded that each atom of chromium cannot be associated with more 


27 The Colloid Chemistry of Basic Chromic Solutions. F. L. Seymour-Jones. Ind, Eng, 


Chem. 15 (1923), 75. : 
*8 Proc, Am, Acad. Arts Sci. 39 (1903), 1, 


308 THE CHEMISTRY OF LEATHER MANUFACTURE 


than two positive charges and probably with not more than one. Assum- 
ing that the sulfate ion alone migrates anodically with a mobility of 
70, the mobility of the chromium group is 41 assuming a single charge 
and 243 assuming a double charge, but the latter figure appears much 
too high. They suggest that the cation may be Cr(OH).*, which 
Siewert *° and Whitney *° had shown to be the most pees cation in 
boiled solutions of chromic chloride or nitrate. 

The author’s view of the mechanism of chrome tanning is as fol- 
lows: Although the degree of ionization of the carboxyl groups of 
the protein, in which a hydrogen ion passes into solution, may be- 
come extremely small with increasing acidity, it never becomes zero. 
This means that, even if the electrical charge on the protein structure 
is predominantly positive, there still remain a relatively small number of 
negatively charged groups scattered throughout this structure. 
Cr(OH).*, or ions of a similar nature, diffuses into the jelly com- 
posing the fibers of the skin and combines with these negatively 
charged groups wherever encountered. Having neutralized the elec- 
trical charges on each other, both the collagen and chromium groups 
become capable of ionizing further, the chromium group giving off 
another hydroxide group and the collagen a hydrogen ion. With a 
repetition of this process, all three bonds of the chromium become 
united directly with the collagen structure. The fundamental as- 
sumption underlying this view is that however small may be the con- 
centration of negatively charged groups in the collagen structure under 
the conditions of tanning, it is very much larger than would result 
from the dissociation of the chromium compound of collagen. 

This theory is not antagonistic to the Procter-Wilson theory of 
vegetable tanning, but actually supplements it. In vegetable tanning, 
the tannin probably attaches itself to the amino or other basic groups 
of the protein structure; in chrome tanning, the chromium attaches 
itself to the carboxyl or other acid groups. This is corroborated by 
the fact that chrome tanning apparently does not lessen the capacity 
of the skin for combination with vegetable tanning materials, or vice 
versa. Wood *! found that plates of gelatin tanned with chromium were 
capable of combining with as much tannin as before the chrome 
tanning, suggesting that the chromium and tannin are not attached 
to the same groups in the protein structure. This is not in accord with 
the Thompson-Atkin theory, in which both chromium and tannin would 
be attached only to the positively charged groups of the protein. That 
the collagen undergoes a chemical change in chrome tanning is proved 
by the fact that it loses its property of being convertible into gelatin 
by contact with boiling water. 

20 Ann, Chem. Pharm. 126 PAE 86. 

LES oy peck Chem. 20 (1896), 


A 51'The Compounds of Gelatin abe Tannin. J. T. Wood. J. Soc. Chem, Ind. 27 (1908), 
354. 


Chapter I5. 
Other Methods of Tanning. 


Of the innumerable substances capable of combining with. collagen, 
the only ones classed as tanning agents are those whose compounds 
with collagen are but little dissociated, imputrescible, incapable of swell- 
ing greatly in water, and stable under ordinary condition and which 
cause the skin fibers to lose their tendency to glue together during 
drying. The suitability of a material as a tanning agent naturally de- 
pends upon its availability and cost, the simplicity of its use, and the 
properties of the leather which it yields. The high quality and yield 
of leather given by the natural vegetable tanning materials and the 
simplicity of tanning with chromium compounds make these two classes 
of materials the favorites in leather manufacture. Other materials, 
however, find a place in the manufacture of special types of leather, 
either alone or in conjunction with vegetable or chrome tanning 
materials. 


Combination of Chrome and Vegetable Tanning. 


Attempts to combine the advantages of both chrome and vegetable 
tanning have met with some success for certain classes of leather. 
During the war there was a demand for vegetable tanned leather for 
shoe uppers, but the length of time required for the tanning of hides 
to produce this leather made it impossible to meet the demand. The 
tanning could be done quickly enough with chrome liquors, but the 
resulting leather was not suitable. It was found, however, that the 
leather could be made to serve the purpose tolerably well by giving it 
a partial tannage in vegetable tan liquors after it had been completely 
tanned with chrome. The best example of this was the so-called chrome 
retan army upper leather, a vertical section of which is shown in 
Fig. 148. This leather was made from cow hide and was first tanned 
with chrome liquor and then hung in vegetable tan liquor for a few 
days, or until the tan liquor had penetrated more than half of the 
thickness of the hide. The hide was then split down to the required 
thickness, although the splitting was done before the retanning in 
some cases. The lightly colored band running across the lower half 
of the picture represents the inner layer of the hide to which the 
tan liquor did not penetrate. It appears nearer to the flesh surface 
only because the hide was split after retanning. It will be noted 
that the fibers in the retanned portions are larger than those in the 
pure chrome layer. At the lower left hand corner, a fiber can be 


309 


Fig. 148.—Vertical Section of Chrome-Retan Army Leather. 
(Cow hide.) 


_ Location: butt. Eyepiece: none. 

Thickness of section: 40 p. Objective: 16-mm. 
“Stain: none. Wratten filter: K3-yellow. 
“'Tarinage: chrome plus vegetable. Magnification: 54 diameters, 


310 


OTHER METHODS OF TANNING SLi 


seen running from the retanned to the pure chrome layer; its size 
decreases noticeably. 

The advantage of the preliminary chrome tanning lies in the speed 
with which the subsequent vegetable tanning operation may be carried 
out. The chrome tanned hides may be put at once into liquors stronger 
than usual, which hastens the rate of penetration, and it is not neces- 
sary to wait for complete diffusion, since the chrome tanning of 
the middle layer renders it imputrescible. ‘The vegetable retanning 
adds firmness to the leather and also reduces the sulfuric acid content 
of the chrome leather to about half its normal value. i 


Alum Tanning. 


The use of aluminum salts for tanning skins dates back to. very 
early times and is still applied to some kinds of leathers and in the 
manufacture of some furs. It never gained the popularity accorded 
to chrome tanning, however, because the initial compounds formed be- 
tween collagen and aluminum compounds are much less stable than the 
ones formed with chromium compounds. Assuming that aluminum 
hydroxide is a stronger acid than chromium hydroxide, our theory of 
chrome tanning offers a plausible explanation of this fact. It would 
then be more difficult for all three bonds of the aluminum to combine 
with collagen. Where only two bonds of the aluminum are combined 
with collagen, we should expect the resulting compound to be very 
much more readily hydrolyzed than where all three bonds are com- 
bined. After drying and storing for months, alum leathers become 
much more stable and resistant to washing, suggesting that the com- 
bination of the third bond of the aluminum with collagen requires a 
long time. 

Some support is given to this view by the work of Lumiere and 
Seyewetz! and of A. and L. Lumiére* on gelatin. They studied the 
combination of both chromium and aluminum salts with gelatin and 
found maximum limiting values for the extent of combination of 
metal with protein, under the conditions of their experiments. They 
found that 100 grams of gelatin combine with a maximum of 3.6 grams 
of Al,O, or 3.2 to 3.5 grams of Cr.O;. Taking 768 as the equivalent 
weight of gelatin and assuming that it combines with all three bonds 
of the chromium or aluminum, we calculate that 100 grams of gelatin 
would combine with 3.30 grams of Cr.O3 or 2.21 grams of AI,Os. 
The-agreement in the case of the chromic oxide is good, but the ob- 
served amount of combined alumina is about fifty per cent greater 
than that calculated, which, however, would be expected if only two 
bonds of the aluminum combined with the gelatin. 

In any case alum tanning must be conducted very differently from 
chrome tanning. Chrome leather is resistant to washing immediately 
after tanning, but if freshly tanned alum leathers are washed, aluminum 
salts are given up and the skins swell as in dilute acid. 


1 Composition of Gelatin Rendered Insoluble by Salts of Cl i ioxi 
Lumiére and A. Seyewetz. Bull. soc. chim, 29 (1903), 1077. Bara atm eed oe 
2 Action of Alums and Aluminum Salts on Gelatin. A. and L. Lumiére. Brit, J. Phot. 
53 (1906), 573. . 


Fig. 149.—Vertical Section of Persian Lamb Fur. 


Location: (?). Eyepiece: none. 

Thickness of section: 30 u. Objective: 16-mm. 

Stain: solid brown. Wratten filter: H-blue green. 
Tannage: alum. Magnification: 90 diameters. 


312 


OTHER METHODS OF TANNING 313 


In practice the skins are tumbled in a solution of basic aluminum 
sulfate and enough sodium chloride to prevent undue swelling of the 
skin. It is sometimes desirable to add enough sodium bicarbonate to 
the liquor, after the alum has completely penetrated the skins, to 
bring it to the point at which precipitation just begins. After a 
few hours longer, or next day, the skins are rubbed with a mixture 
of egg yolk, cottonseed oil, and flour, but are not washed. They 
are then allowed to dry thoroughly. Sometimes the egg yolk mixture 
is added to the tanning bath. The skins are kept in the dried state 
for weeks, or months, to give the aluminum time to become permanently 
fixed. The skins are then soaked in water to remove the salt and are 
fatliquored and colored and finished according to the kind of leather 
required. 

In tanning skins for furs, it is customary to work the alum solu- 
tion into the skin from the flesh side, supplemented with oils to 
keep the skin soft. Work of this kind is usually done by hand and 
requires some skill in order to get the best results. Fig, 149 shows a 
section of alum tanned fur, known as Persian lamb. Skins for this 
fur are from specially bred lamb slinks. In this particular skin, the 
wool and skin were dyed black with logwood and iron salts. 


Iron Tanning. 


The tanning properties of ferric salts have been known for more 
than a century, but attempts to manufacture iron tanned leathers have 
met with so many difficulties that the subject still remains in the ex- 
perimental stage at this late date. Procter*® points out that part of 
the trouble is due to the fact that iron salts are oxygen carriers. 
Ferric salts readily give up oxygen to certain kinds of organic matter, 
being reduced to the ferrous state, in which they take up oxygen from 
the air, under suitable conditions, returning to the ferric state. In 
this way a slow oxidation of the leather occurs, with consequent 
deterioration. ) 

Jettmar* found the greatest difficulty in iron tanning to be that 
of proper neutralization of the leather. If chrome leather contains 
too high a proportion of sulfuric acid, it becomes very brittle upon 
drying and assumes a.very dark green color. But the proper degree 
of neutralization of chrome leather is a comparatively simple matter. 
In attempting to neutralize iron tanned leather, Jettmar found that 
the iron salts became colloidally dispersed and were washed out of 
the leather. Apparently the neutralization is necessary to bring about 
a permanent combination between iron and collagen, but the iron com- 
pounds pass into the colloidal state, upon neutralization, before they 
have had the opportunity to combine with the collagen. Since col- 
loidal ferric oxide carries an electrical charge of the same sign as 
that on the collagen in acid solution, there would be no tendency for 


® The Principles of Leather Manufacture. 2nd Edition, p. 275. 
“Iron Tannage. J. Jettmar. Cuir. 8 (1919), 74, 106 


314 THE CHEMISTRY OF LEATHER MANUFACTURE 


the two to combine. This difficulty was partly overcome by using a 
strong solution of neutral salt to prevent the formation of colloidal 
ferric oxide during the neutralization. The iron tanned leather was 
improved in quality by retanning it with formaldehyde. 

Rohm * patented the use of the salt FeSO,Cl for tanning. It is 
obtained by the action of chlorine upon ferrous sulfate. Just why 
this salt should have unusual tanning properties is not made clear. 

A good summary of much of the work done on iron tanning is con- 
tained in a paper by Jackson and Hou,°® who carried on an extensive 
investigation of the tanning properties of iron salts. They pointed 
out that investigators generally seem to have the idea that the salt 
responsible for the tanning action has the formula FeOHSOs,, whereas 
this salt is not stable in solution, but invariably gives a precipitate 
of hydrated ferric oxide. They attribute the brittleness usually as- 
sociated with iron tanned leathers to improper tanning, resulting from 
the precipitation of this hydrated ferric oxide. They showed that 
ferric sulfate is much more readily precipitable than chromic sulfate 
by diluting a solution of the two. With increasing dilution, there was 
a progressive precipitation of iron, but the chromium remained in 
solution. 

Jackson and Hou prepared iron tanned leather which they were con- 
vinced compared favorably with other mineral tanned leathers. Its 
character seemed to lie between that of alum and chrome leathers. It 
would not stand the action of boiling water, but would shrink when 
brought into contact with water having a temperature above 75° C. 
They summarize the chief factors in their findings as follows: The 
iron salts must be kept in the ferric state by using an excess of a 
- proper oxidizing agent and by means of an after oxidation. During 
tanning, the acidity of the liquor must be so adjusted as to give a basic 
salt of iron in which the ratio of equivalents of hydroxide groups to 
equivalents of acid radical is never less than 1:5 nor more than 1:3. 
Neutralization must be effected so gradually as to effect a uniform 
fixation of iron throughout the skin. The leather must be dried 
before subsequent treatment in order to minimize the reactions between 
free iron and materials used later that would give the leather an 
undesirable color. 

During the war the scarcity of tanning materials forced Germany 
to investigate every available source, including iron salts, and had 
the war continued long enough, she might have been forced to produce 
iron tanned leathers on a large scale. But, unless iron leathers are 
produced which are at least the equal of chrome leather, it is doubt- 
ful that they will ever be made on an extensive scale, except in cases 
of emergency, because the total cost of tanning material used in 
making chrome leather is small compared to the loss in selling price 
that would result from even a very small depreciation in quality of 
the leather. 7 : 


5 British Pat, 146,214, June 26, 1920. 


®Tron Tannage. D. D. k 
peer a are Jackson and T. P. Hou. J. Am. Leather Chem, Assoc, Baa hee, 


OTHER METHODS OF TANNING 315 


Tanning with Silicic Acid. 


Like tannin, colloidal silicic acid’ is negatively charged and pre- 
cipitates gelatin from solution. In 1862 Thomas Graham’ studied 
the precipitation of gelatin by silicic acid and reported as follows: 
“Silicate of gelatine falls as a flaky, white and opaque substance, 
when the solution of silicic acid is added gradually to a solution of 
gelatine in excess, The precipitate is insoluble in water and is not 
decomposed by washing. Silicate of gelatine, prepared in the manner 
described, contains 100 silicic acid to about 92 gelatine. In the humid 
state the gelatine of this compound does not putrefy. When a solution 
of gelatine was poured into silicic acid in excess, the co-silicate of 
gelatine formed gave, upon analysis, 100 silicic acid with 56 gelatine.” 

This experiment inspired Hough ® to experiment with silicic acid 
as a tanning agent. He found that a purified silica sol is much too 
easily precipitated to be of any value as a tanning agent, but finally pre- 
pared a solution of silicic acid capable of tanning by adding a thirty- 
per cent solution of sodium silicate to a thirty-per cent solution of 
hydrochloric acid until the concentration of free acid was reduced 
to decinormal. If the acid is poured into the sodium silicate solution, 
the silica will be precipitated when the neutral point is approached ; 
the silicate must always be poured into the acid solution whose strength 
must not be allowed to fall below decinormal. The tanning proceeds 
a little faster than is the case with vegetable tanning, light skins being 
fully tanned in from 3 to 5 days, and heavy bull hides in about a 
month. ‘The leather usually contains from 17 to 24 per cent of silica; 
in fact one of the difficulties of the process is to prevent too great a 
combination of silica with the skin protein. The leather is pure white 
and may be finished like ordinary chrome leathers. 

Hough attributes the failure met with in attempting to combine 
silica and vegetable tanning to the fact that both the silica and tannin 
are negatively charged and tend to combine with the same amino 
groups of the collagen structure. On the other hand, good leathers 
were produced by a combined silica and alum tannage, probably be- 
cause the alum attaches itself to the carboxyl groups of the collagen. 
The presence of alum, however, seemed to retard the tanning, pos- 
 sibly on account of the condensation of aluminum silicate on the surface 
of the skins hindering the penetration. But by giving the skins a pre- 
liminary chrome tannage and then putting them into the silica liquors, 
the raté of tanning was increased and the final leather had greater 
solidity and firmness. : 

In discussing the evolution of the different methods of tanning, 
Thuau ® states that one serious fault with silica tanning is that the 
leather, after keeping for a few months, tears very easily, probably 
because of the action of the silicic acid on the leather fibers. If 

™J. Chem. Soc. 15 (1862), 246. 

® Tanning with Silica’ A. T. Hough. Cuir. 8 (1919), 209, 257, 31 


. '* Evolution of the Different Methods of Tanning. U. J. Anan Cuir. 9 (1921), 10, 
0, 102, ; 


316 THE CHEMISTRY OF LEATHER MANUFACTURE 


this difficulty could be overcome, he predicts that the process might 
supplant chrome tanning. 


Miscellaneous Mineral Tannages. 


Sommerhoff ?° claims to have discovered that certain insoluble sul- 
fides, silicates, hydroxides, and phosphates of the heavy metals have a 
marked tanning effect on skin, when freshly precipitated or colloidally 
dispersed. In one experiment, copper sulfate was precipitated with 
disodium phosphate. “he jelly-like precipitate was filtered off and 
then suspended in water and shaken with a piece of hide, which be- 
came completely tanned in about two hours. ‘The leather contained 
about 13 per cent of ash. Whether Sommerhoff really obtained a 


_ tanning action is difficult to determine from his paper, but the subject 


seems not to have been carried very far. 

Garelli ++ found that the tanning properties of cerium chloride are 
much like those of aluminum salts, provided the solution used for 
tanning 1s made sutficiently basic and dilute. Wiauth properly adjusted 
conditions, he obtained a pliable, white leather containing about 9g per 
cent of Ce,03. Garelli and Apostolo’* were not so successful in 
attempting to tan skin with salts of bismuth. lf any compound between 
the bismuth and collagen were formed, it was unstable, the skin re- 
turning to the raw condition upon washing in cold water. 

‘he experiments of Apostolo ‘* seem to indicate that freshly pre- 
cipitated suitur has tanning properties. He added a small amount of 
lactic acid to a concentrated solution of sodium thiosulfate, which be- 
came turbid, due to the precipitation of sultur. Into the turbid solu- 
tion he put a piece of skin, which apparently absorbed all of the free 
suitur. ihe skin was withdrawn long enough to add a little more acid 
to berate more sultur. ‘Lhe skin was returned to the liquor and it 
again absorbed all ot the sultur in suspension. ‘his was repeated a 
llumver ot times, care being taken to avoid using acid in excess of the 
amount of sodium thiosuitate present. ‘he leather obtained is de- 
scribed as white, extraordinarily soft, and of beautiful appearance. 
it did not swell when lett for 24 hours in cold water, and when dried 
and stretched again had lost none of its quality. ‘The leather gave 
up only I per cent of sulfur to carbon disulfide and this seemed to 
have been merely mechanically held, as the remaining skin seemed still 
to be fully tanned, and contained 2.5 to 3.5 per cent of sulfur. . The 
leather was decomposed, however, when brought into contact with hot 
water. 

imeunier and Seyewetz ‘* made the rather startling discovery that 
chiorme and bromine have tanning properties. Plates of gelatin are 


©The Tannage of Hide by Means of Insoluble Metallic Jellies. E. O. Sommerhoff. 
Collegium (1913), 381. 

~ Laniung vy waeans of Cerium Salts. F, Garelli, Collegium (1912), 418. 

% Action ot Bismuth Salts on Skins. F. Garelli and C. Apostolo. Collegium (1913), 
422. ; ‘ : ss 

18 Tanning of Hides with Freshly Precipitated Sulfur. C, Apostolo, Collegium (1913), 
420. 
14 New Studies Dealing with the Tanning of Gelatin and of Skin. L. Meunier and A. 
Seyewetz. Collegium (1911), 373. 


OTHER METHODS OF TANNING SUT) 


rendered insoluble by contact with aqueous solutions of either bromine 
or chlorine, in the presence of salt to prevent swelling. The gelatin 
evidently combines vigorously with either element. Raw skin was 
found to be affected similarly. Iodine had no such effect on either 
gelatin or skin. The authors suggested the use of chlorine or bromine 
as a preservative for skin and also as a tanning agent to be used 
prior to tanning by other methods for all kinds of skins. The leather 
obtained is absolutely imputrescible and resistant to cold, but not boiling, 
water. 


Tanning with Oils. 


One of the commonest examples of oil tanned leather is the ordi- 
nary chamois leather. This is made from the reticular layer of sheep 
skin, which is split from the grain layer so that the two may be tanned 
separately for very different purposes. The flesh splits are soaked 
with cod oil and pummeled in specially designed machines in order to 
assist the penetration of the oil. A combination of oil with the skin 
protein takes place with the evolution of acrylic aldehyde and other 
pungent products and the development of a considerable amount of heat. 
Oxidation of the oil occurs simultaneously. The pummeling, or stock- 
ing, is stopped occasionally and the skins are spread out to cool off, 
after which the stocking is continued. The completion of the process 
can only be determined by practice. After tanning, the splits are 
soaked for a few hours in warm water and then pressed to remove 
uncombined oil, which is sold under the name moellon degras. The 
oil still adhering to the skins is removed by washing with a solution 
of sodium carbonate. The skins are then bleached in strong sunlight. 

The nature of the combination of oils with collagen has been studied 
by Fahrion*® and by Meunier.*® According to Fahrion, the un- 
saturated, free fatty acids are the active tanning agents of the fish oils 
in chamoising. The active acids have at least two double bonds and 
upon oxidation they assume a peroxide structure represented by the 
formula 


ee CH... Sea) . . COOH 
| | 


| 
O——O O——O 


Representing collagen by the simplified formula R’NHz, the combina- 
tion of oxidized fatty acid and collagen, according to Fahrion’s view, 
would yield the following compound: 


RCH —CH—.. .—CH—CH—.:.COOH 


| | | 
HO OH One © 


fi Ss 
R/NH  HNR’ 


18 Theory of Leather Formation. W. Fahrion. Z. angew. Chem, (1903), 665; (1911), 
2083. : 
16 Modern Theories of the Various Methods of Tanning. L. Meunier. Chimie & in- 
dustrie 1 (1918), 71, 272. 


310, HE CHEMIST he OF LEATHER MANUFACTURE 


He considers that this combination 1s followed by a conversion into 
the lactone 
CG imo asain eat 
R—CH—CH—.. ey eam ee 
| 
OH O O 


va & 
R’NH HNR’ 
and that only a portion of the peroxide acids enter into combination 


with the protein matter, the rest being converted by molecular re- 
arrangement into hydroxy-acids, thus 


SOC cre 
| | a — 0a 
genni G OH 


These hydroxy-acids are then converted into lactones which are re- 
tained by the fibers, being resistant to the alkaline washing which 
completes the manufacture of chamois leather.. The aldehydes formed 
in the process probably also exert a tanning action on the skins. 

According to Meunier, the tanning power of fish oils is due to 
the presence of free fatty acids possessing four double bonds of which 
at least two are capable of combining with oxygen to give the peroxide 
structure shown above. Thus if a skin, previously dehydrated with 
alcohol, be treated with an alcoholic solution of oleic acid, a soft 
leather is obtained, but after draining off the alcohol to remove the 
excess of oleic acid, the skin does not show any sign of being tanned 
when placed in water. Substituting the fatty acids of rape oil for 
oleic, a yellow leather somewhat more resistant to the action of water 
is obtained. These acids belong chiefly to a series possessing two 
double bonds and include a little linolenic acid, with three double 
bonds. Substituting the fatty acids of linseed oil, rich in linolenic 
acid, the leather obtained is still more resistant to water, but is not the 
equal of that made from the fatty acids of cod oil possessing four 
double bonds. 


Tanning with Aldehydes and Quinones. 


Payne and Pullman‘ patented the use of formaldehyde as a 
tanning agent in 1898. Since then a number of investigators have 
studied the action of various aldehydes upon gelatin and skin protein. 
Some aldehydes, like benzaldehyde, show little or no tanning power. 
The use of formaldehyde has frequently been suggested as a means 
of preparing skins for tanning with vegetable tanning materials by the 
newer rapid methods, in which stronger tan liquors are used. The 
formaldehyde adds very little weight to the leather, but its combina- 

17 British Pat. 2872, Feb. 4, 1808. 


OTHER METHODS OF TANNING 319 


tion with skin prior to vegetable tanning lessens the astringency of 
the tan liquor and increases its rate of diffusion into the skin. ) 

Hey 1® observed that formaldehyde has tanning properties only in 
solutions having a pH value greater than 4.8 and that the best prac- 
tical results are obtained when the pH value lies between 5.5 and 10.0. 
At higher pH values the skin becomes swollen and the surface be-. 
comes almost impermeable to the formaldehyde not combined with 
the skin at the surface. Meunier suggests that oxidation of the skin 
proteins, affecting the amino groups, precedes combination with either 
aldehydes or quinones, whose tanning properties were discussed in 
Chapter 13. When quinone combines with skin protein, part of the 
uncombined quinone is reduced to quinol, which Meunier attributes 
to the oxidation of protein. 

Meunier also studied the action of gallic acid, naphthols, quinol, 
pyrogallol, diaminophenol hydrochloride, and resorcinol upon gelatin. 
When these substances are dissolved in dilute solutions of sodium 
carbonate and exposed to air, they act slowly upon gelatin, rendering 
it insoluble in boiling water. Without access to the air, however, 
this action is not obtained. 


Tanning with Syntans. 


In vegetable tanning, in slightly acid solution, combination takes 
place between positively charged protein and negatively charged colloidal 
particles of tannin. A distinct advance in the science of tanning was 
made when Stiasny *® discovered that water soluble products can be 
obtained by mixing phenolsulfonic acids with formaldehyde, under 
the right conditions, in which the particles formed are negatively charged 
in acid solution, precipitate gelatin dispersions, and possess marked 
tanning properties. Equal parts of cresol and sulfuric acid are heated, 
with stirring, for two hours, the temperature being kept at about Io a Oe 
The mixture is then cooled to 35° C. and 1 molecule of formaldehyde 
is added for each molecule of cresol present. The important point 
to be observed is that the formaldehyde must be added very slowly 
and the temperature must not be allowed to rise above 35° or the 
ordinary insoluble products may be obtained. 

According to Grasser,?° the reaction proceeds as follows: 


OH OH OH 
x YS PEE UES 
2 Micon |" | 


| | + H.0. 
CH; NEG (cH. EG fs 
HSO; HSO; HSO,; 


Resides cresols, naphthalenes and higher hydrocarbons are also used 
in the preparation of these synthetic materials, now commonly called 
18 Formaldehyde Tannage. A. M. Hey. J. Soc. Leather Trades Chem. 6 (1922), 131. 

12 A New Synthetic Tannin. E. Stiasny. Collegium (i913). 142: 


20 Synthetic Tannins. G. Grasser. English translation by F. G. A, Enna. Crosby Lock- 
wood & Son, London (1922). 


320. THE CHEMISTRY OF LEATHER MANUFACTURE 


syntans. For an interesting discussion of these, the reader is referred 
to Grasser’s recent book. 

Raw skin can be tanned by simply immersing in a pure solution of 
these syntans, provided the concentration and acidity are properly 
adjusted. With, decreasing acidity, they seem to lose their tanning 
properties. A typical commercial syntan preparation examined by the 
author showed a titrable acidity of 0.65 gram equivalents per liter 
and a pH value of 0.63. Grasser found that concentrated solutions of 
Neradol D, the original syntan, actually cause a gelatinization of raw 
skin, which might have been expected from its high acidity. But when 
used at sufficient dilution, no ill effects were observed at all and a 
tough, white leather was obtained. Since syntans add much less weight 
to skin than natural vegetable tanning materials, they are seldom 
used: alone, but appear to have valuable properties when used in con- 
junction with other materials for certain kinds of leather. | 

Like formaldehyde and quinone, syntans act upon skin in such a 
way as to lessen the astringent effects of strong tan liquors upon it. 
Syntans, having a lower molecular weight than tannins, diffuse into 
the skin at a much greater rate and also combine with it. This partial — 
tannage lessens the rate of combination of the tannin with the skin, 
thereby increasing its rate of diffusion. The syntan solution may be 
used as a drench for deliming the skins prior to vegetable tanning or 
may be added directly to the tan liquors. Like any strongly acid 
material, however, it must be used with caution and proper control. 

The high acidity of ordinary syntan solutions makes them suitable 
‘for the bleaching of leather, the color of the leather becoming lighter 
with increasing acidity, or decreasing pH value, within limits. 

Another valuable property of these syntans is their power to effect 
the solution of phlobaphenes and other difficultly soluble tannins. Gras- 
ser found that the phlobaphenes of quebracho were rendered soluble 
by solutions of Neradol D or sodium phenolsulfonate, but not by the 
free sulfonic acid. The action of the sodium salt, however, may be 
attributed at least partly to the pH value of its solution, since the 
author found phlobaphenes to be very soluble at pH values above 7. 
Since the phlobaphenes are probably oxidized tannins, it is possible 
that the syntans exert a reducing action upon them, but the real nature 
of the action is not known. 

The method of determining the extent of penetration of Neradol D 
in tanning is to wash a strip of the skin, make a cutting, wash the 
cut portion, and then treat it with a few drops of dilute ferrous 
ammonium sulfate solution, which color the tanned layers a deep blue. 

Another material that should be mentioned in connection with 
syntans is the waste sulfite liquor from the paper mills, known as 
sulfite cellulose. This material, purified for tanning purposes, is sold 
under the name of spruce extract. The active tanning agent of this 
material appears to be the free lignosulfonic acids. Spruce extract 
possesses undoubted tanning properties, but does not yield a satis- 
factory leather when used alone in the ordinary way. But when mixed 


OTHER METHODS OF TANNING 321 


with other materials, such as quebracho extract, it acts much like 
many natural vegetable tanning materials. Its very low costs finds 
it a place as a filler in the manufacture of sole leather. Hill and 
Merryman 2! describe a method of increasing the filling properties 
of spruce by the use of syntans. Sole leather is first soaked in a 
concentrated solution of spruce and then drummed with a solution of 
syntan. They claim that the spruce is precipitated inside of the leather _ 
by the syntan, greatly increasing the weight and at the same time 
brightening the color and lessening the need for bleaching. 

The chemist has almost unlimited possibilities before him in the 
development of new materials to supplement the ever diminishing supply 
of natural tannins. 


21 Some Applications of Synthetic Tanning Materials. J. B. Hill and G. W. Merryman. 
J. Am. Leather Chem. Assoc. 16 (1921), 484. 


Chapter 16. 


Finishing and Miscellaneous Operations. 


The mere conversion of raw skin into leather does not, as a rule, 
make it suitable for the various purposes for which leather is used. 
For some kinds of leather, more work is required in the operations 
following the actual tannage than in the tanning and all preceding 
operations put together. Each of the innumerable kinds of finished 
leather requires a special series of operations after tanning and nothing 
short of an encyclopedic work could adequately treat the details of 
operations, even for the commoner leathers. For this reason, the fol- 
lowing brief treatment is limited to the fundamental principles under- 
lying the few general processes common to large classes of leathers. 


Bleaching. 


In vegetable tanning there is often a deposit of phlobaphenes or 
of ellagic acid on the surfaces of the leather which, if left there, would 
~ interfere with the coloring of the leather, giving rise to irregularities. 
It also frequently happens that the color of the leather becomes dark 
through oxidation and this may not be entirely uniform over the sur- 
face of the leather. Bleaching is a process designed to give the leather 
a lighter and more uniform color before it is sent to be dyed and 
fatliquored. It usually consists in giving the leather a bath first in a 
dilute alkaline solution and then in an acid one, sodium carbonate and 
sulfuric acid generally being used for the purpose. . 

When the leather is soaked in dilute sodium carbonate solution, 
the phlobaphenes and ellagic acid pass into solution, being soluble at 
pH values greater than 7. At the same time the tannins are stripped 
from the surfaces of the leather; it was shown in Chapter 13 that this 
stripping action proceeds at an increasing rate as the pH value is 
increased above 8. The sodium carbonate solution is usually allowed 
to act for only Io or I5 minutes and the leather is then rinsed to 
free it from the soluble products on the surface. It is then treated 
with a dilute solution of sulfuric acid, which checks the action of 
the sodium carbonate and at the same time lightens the color of the 
remaining fixed tannins by lowering the pH value, the color being a 
function of pH value, as pointed out in Chapter 11. In this way the 
grain surface is cleared and the color brightened. After the bleach- 
ing, it is customary to replace the tannin lost from the surface by 
giving the leather a short tannage in clear tan liquor. 

322 


FINISHING AND MISCELLANEOUS OPERATIONS 323 


The use of sulfuric acid in bleaching leather has been condemned 
on the ground that it slowly destroys the leather, if not subsequently 
removed. When sulfuric acid is present in vegetable tanned leather 
in excess of I per cent of the weight of the leather, the leather may 
look all right for two or three years, but gradually deteriorates and 
in time will become as tender as blotting paper. The tanner usually 
takes care that the amount of acid used is not excessive or counter- 
acts it by using an alkaline fatliquor, which neutralizes any acid 
left in the leather. Other bleaching agents sometimes employed are 
sodium bisulfite, organic acids, and syntans. 


Stuffing and Fatliquoring. 


Although the tanning of skin lessens the tendency for the fibers 
to glue together upon drying, it does not lubricate them so that they 
slip easily over one another. In fact, when leather is dried after 
tanning, without further treatment, it is usually very stiff and will 
crack upon bending sharply. In order to give it the desirable soft- 
ness and pliability and to increase its tensile strength and resistance 
to tearing, oils and greases are incorporated into it to lubricate the 
fibers. 

The amount of oil added to leather varies greatly according to 
the use to which the leather is to be put. In sole leather, where 
stiffness is desired, only 2 or 3 per cent of sulfonated oil is used and 
this is often added along with the concentrated tan liquor or other 
material used to fill and weight the leather. Waxed leathers, used for 
waterproof shoes, may contain as high as 30 per cent of oils, waxes, 
and stearin. 

The direct application of oils and greases to leather, either by 
hand or by drumming in the molten greases, is known as stuffing 
and is used where it is desired to incorporate a large amount of grease 
into the leather. Where only a small amount of oil is desired in the 
leather and it is essential to have it fairly uniformly distributed, 
it is best to apply the oil in the form of an emulsion, in which case 
the process is known as fatliquoring. 

In dry stuffing, the dried leather is treated with hot, molten 
greases, which penetrate rapidly. This method is suitable only where 
it is desired to have a finished leather containing upwards of 20 
per cent of grease. For leathers with less grease, it is preferable 
to apply the greases to the wet leather. In belting leather, for ex- 
ample, it is customary to swab a mixture of cod oil and tallow over 
the surfaces of the leather while thoroughly wet. The leather 1s 
then hung in a drying chamber in which the temperature is gradually 
raised. As the water passes out of the leather, the oil and tallow 
diffuse in. When a small amount of oil is rubbed onto dry leather, 
the surface tends to remain oily, but when applied to wet leather, 
the surface of the leather after drying is not oily. For this reason 
it has been assumed that the evaporation of water from the leather 
draws the oil into the leather, but this view has been contested by 


324 THE CHEMISTRY OF. LEATHER MANUFACTURE 


Moeller? who considers the action of oils upon wet leather as similar 
to that of cod oil in chamoising. According to his view, the vegetable 
tanning material causes an oxidation of the unsaturated fatty acids, 
as indicated by the formation of oxidized acids and the simultaneous 
disappearance of water from the leather, water presumably being 
essential to the reaction. The oxidized fatty acids then act as a 
tanning agent. When dry leather is oiled, this action proceeds so 
slowly that the leather always remains oily, whereas wet leather, after 
oiling, dries without retaining the oily condition. 

The effect of grease in increasing the tensile strength of leather 
was demonstrated in a special investigation by Whitmore, Hart, and 
Beck,? who found that a petrolatum-paraffin mixture was quite as 
effective as the commoner cod: oil-tallow mixture. Bowker and 
Churchill ? showed that grease in excess of a certain amount does not 
add to the tensile strength, but may actually decrease it. This agrees 
with some observations made by the author in the stuffing of strap 
leather, in which the tearing strength decreased with increase of grease 
content above 21 per cent. 

Light leathers, such as those for shoe uppers, are fatliquored 
rather than stuffed because fatliquoring leaves the grain surface in a 
much better condition for coloring. The common emulsifying agents 
are sulfonated oils, soaps, and moellon degras, the by-product from 
the manufacture of chamois leather. For a review on the literature 
of emulsions, the reader is referred to the paper by Thomas.t In 
the manufacture of chrome leathers, it is sometimes necessary to neu- 
tralize a portion of the sulfuric acid of the leather before applying 
the fatliquor, which is done by drumming the leather in a calculated 
quantity of sodium bicarbonate or borax solution. A fatliquor may 
be made simply by shaking a mixture of sulfonated neatsfoot oil, 
neutral neatsfoot oil, and hot water. Or the sulfonated oil can be 
replaced by soap and moellon degras. Van Tassel *® proposed the use 
of stearamid as an excellent emulsifying agent in making fatliquors. 
Larger quantities of fatliquor are used for vegetable tanned leathers 
than for chrome. A finished chromé leather generally contains from 
4 to 8 per cent of oil against from 10 to 15 per cent for vegetable tanned 
upper leathers. ) 

The skins are thoroughly wet with hot water and drummed with a 
small volume of hot fatliquor for. about half an hour, at the end of 
which time practically all of the oil should be separated from the 
solution. It has often been thought that the oil globules actually 
penetrate the leather during fatliquoring, but Albert F. Gallun {renin 


eT 
Wl aS of Ola: Crecien aa Beaten Ge Penne, Cheat ae 
Harness Leather. R. C. Bowker and J. B. Churchill. Bureau Of Standen ds eee 
Pea die ees Literature of Emulsions. A. W. Thomas. J. Ind fs, : 
ay Nes Emulsifying Agent and Its Pee i ee 3 : were 
Jr. J. Am, Leather Chem. Assoc. 9 (1914), 236. y Practice. E. D, Van Tassel, 


FINISHING AND MISCELLANEOUS OPERATIONS 325 


an unpublished work, showed that they do not do so. By splitting 
skins into layers immediately after fatliquoring, he found by analysis 
of each layer that the oil had not penetrated a measurable distance. 
Upon drying, however, the oil slowly diffuses into the interior and 
tends, in time, to distribute itself uniformly throughout the skin. 
The author split a skin into five layers after it had been dried after 
fatliquoring and found the highest percentages of oil in the outermost 
layers and the lowest in the layer just under the grain surface. This 
is probably due to the fact that the fibers of the flesh side of the 
skin offer more surface to the oil globules than the grain surface and 
hence the greater portion of the oil adheres to the flesh side. 

The discussion of the stability of colloidal dispersions given in 
Chapter 5 may be applied to fatliquoring. The oil globules possess a 
negative electrical charge which gives the surface film of solution 
surrounding each globule an electrical difference of potential against 
the bulk of solution. The condition of leather to be fatliquored may 
be taken as that of leather in equilibrium with a solution having a 
pH value of from 4 to 5. The leather thus carries a positive electrical 
charge. The effect of putting such leather into a fatliquor, although 
similar in some respects to the immersion of skin in a vegetable tan 
liquor, is very different in at least two ways: the oil globules are 
very much larger than tannin particles and are not stable at pH values 
much below 6. The negatively charged globules will tend to combine 
with the positively charged leather and there may actually be some 
combination. But the emulsion is quickly broken up by the soluble 
matter present in the leather. The stability of the emulsion may be 
increased by making it more strongly alkaline, but this must be done 
with extreme caution or the leather will be ruined by overneutralization. 

Experience shows that the more quickly the emulsion is broken up 
during fatliquoring, the greater the difficulty of setting a uniform distri- 
bution of the oil throughout the leather upon drying. For this reason 
the water soluble matter contained in leather to be fatliquored may 
exert a detrimental effect. The soluble tanning matters remaining in 
the leather have a tendency to break up the emulsion and, if present 
in too great an amount, may break the emulsion before the fatliquor 
has had time to serve its purpose properly and the leather will dry hard 
and crack easily. On the other hand, a leather practically free from 
soluble matter takes the fatliquor so well that it may become too soft 
after drying unless a smaller amount of oil is used in fatliquoring 
than is applied to leathers containing much soluble matter. 


Penetration of Dispersions through Grain Surface. 


It is interesting to note how small the particles of a dispersion 
would have to be in order to pass between the fibers constituting the 
erain surface of the leather, without distortion and assuming they 
could pass without coalescing or combining with the leather. Fig. 150 
shows a horizontal section of vegetable tanned calf skin comprising the 
grain surface. The view is that looking down on the uppermost sur- 


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326 


FINISHING AND MISCELLANEOUS OPERATIONS 327 


face which has been cut away from the leather below it at a thickness 
of less than 15 microns. (The exact measurement of thickness could 
not be made because this was the first cutting of the paraffin block 
at 15 microns that included any leather at all and was taken so as 
not to lose the upper surface.) The section was stained for 2 minutes 
in a I-per cent solution of indigo carmine, The specimen was taken 
from a skin about to be fatliquored. 

The average distance between the fibers of the surface is about 
2 microns. If size of particle alone counted, it would merely be nec- 
essary to prepare the dispersion so as to get particles having a diameter 
less than this. The large empty spaces in the figure are the openings 
of the empty hair follicles. In the ordinary course of fatliquoring, 
these probably become filled with oil as soon as the emulsion breaks. 


Fatty Acid Spews. 


Sometimes when finished leathers are chilled, the surface becomes 
coated with a white crystalline deposit resembling a thin film of 
snow. ‘This deposit, known to the trade as spew, usually consists of 
saturated fatty acids having a high melting point. For this reason, 
many tanners try to avoid the use of oils containing much stearin 
or free stearic or palmitic acids. Where sulfonated oils have been 
used, the deposit may contain sulfonated fatty:acids, which are more 
difficult to remove from the surface of the leather than pure stearic 
or palmitic acid. Although the spew does no harm to the leather, 
it ig undesirable because it detracts from its appearance. Its removal, 
however, is a comparatively simple matter and may be effected by 
wiping the surface with a cloth wet with naphtha or with a soap solution. 

Fahrion® found that the splitting of a fat, with the liberation 
of saturated fatty acids, is not the only cause of spew of this kind. 
Glycerides may appear on the surface of leather which have a lower 
melting point than those left in the leather. But when the fatty acids 
are liberated from the spew, they are found to have a higher melting 
point and a greater tendency to crystallize than the fatty acids liberated 
from the glycerides remaining in the leather. A glyceride mixture is 
likely to cause spewing if another mixture can be separated from 
it by fractionation which has a lower melting point, but a higher con- 
tent of saturated fatty acids. He points out that the less the tendency 
of a fat to crystallize, the more suitable it is for purposes of fatliquoring. 

When fatliquors containing saturated fatty acids are used, the 
danger of spewing can be greatly lessened by incorporating in the fat- 
liquor materials like mineral oils or sulfonated castor oil. These 
remain liquid at ordinary temperatures and are solvents for the fatty 
acids and glycerides which form ordinary spews. 

When a piece of finished leather spews, it is often noted that the 
greatest deposits of fatty acids occur where the leather is thinnest. 
This is due simply to the fact that leathers fatliquored in the ordinary 
way have a higher total fat content in the thin parts than in the heavier 


6 Properties of Fat in Leather. W. Fahrion. Gerber 43 (1917), 123. 


328 THE CHEMISTRY OF LEATHER MANUFPACTGRe 


regions. In fact the fat content of the various parts may be taken 
as inversely proportional to the thickness. This is easily explained 
by the fact that the oil globules are deposited only on the surfaces of 
the leather during fatliquoring. Equal areas of leather thus receive 
the same amount of oil. If the butt is twice as thick as the shanks, 
it will receive only half as much oil per unit volume. For the same 
reason, if a very thick skin is fatliquored along with a very thin one, 
the thick one will not get enough oil, while the thin one will get 
more than its share. 

A type of spew less common, but more difficult to remove, than 
that made up of saturated fatty acids is the resinous spew caused by 
the oxidation of unsaturated fatty acids in the leather. If a vegetable 
tanned leather contains much soluble tanning material, the tendency 
for the free tannin to absorb oxygen from the air increases upon fat- 
liquoring due to the resulting increase in pH value. The oxidized 
tarinin seems to give up its oxygen readily to the unsaturated fatty 
acids of cod and similar oils. The higher the pH value of the solu- 
tion in the leather and the greater its content of free tannin and 
oxidizable fatty acids, the greater will be the danger of the formation 
of resinous spews, consisting of oxidized fatty acids. 

Most tanners appreciate that it is undesirable to allow a large 
amount of soluble neutral salts to remain in the finished leather and, 
since such salts are easily washed out during the process, there is no 
good reason why they should not be removed before finishing. Never- 
theless leathers are occasionally found on the market showing a de- 
posit of salt crystals on the surface, resembling spew. It is, of course, 
an easy matter to differentiate between such salt deposits and ordinary 
fatty spews. 

When leather is kept in a cool, damp place for a long time, it 
is apt to be subject to the growth of molds. Sometimes these show 
the light green color of the common mold penicillium glaucum, but 
often they appear like a white, powdery deposit, resembling the spew 
sometimes occurring on leathers fatliquored with sulfonated neatsfoot 
oil. 


Coloring. 


Leather is dyed either before or after the fatliquoring process, 
depending upon the kind of leather produced and the nature of the 
other operations. Natural dyestuffs are still employed for coloring 
some kinds of leather, but have been largely supplanted by artificial 
products. In coloring vegetable tanned leather, basic dyestuffs are 
usually preferred because they combine readily with tannin and give 
more intense shades than the acid dyestuffs. 

When vegetable tanned leathers are to be colored with basic dye- 
stuffs, they must first be washed so that no free tannin will diffuse 
into the dye bath, where it would be precipitated by the dye and 
tend to cause spottiness and discoloration of the leather. The leather 
is sometimes given a short rinsing in sodium carbonate solution in 


FINISHING AND MISCELLANEOUS OPERATIONS 329 


order to free the grain surface from precipitated tannin and to strip 
off any excess of fixed tannin, the object being to get clearer and more 
uniform coloring. The leather is then drummed in a solution of a salt 
of titanium or antimony, such as titanium potassium oxalate or 
antimony potassium tartrate, which serves the double purpose of acting 
as a mordant and of precipitating any remaining free tannin that 
might otherwise diffuse into the dye bath. The leather is then given a 
thorough washing and is then drummed in a solution of the dyestuff. 
Since basic dyestuffs are precipitated by hard waters, where these must 
be used, they should first be acidified with acetic acid, which will 
prevent precipitation. 

An objection to the use of acid dyes is the fact that strong acids 
are necessary to develop the color. The sulfuric acid often used 
exerts a detrimental effect upon vegetable tanned leathers, if not neu- 
tralized after coloring. Where later neutralization is objectionable, 
it is much better to use formic acid to develop the color since this 
acid appears to have no harmful effects whatever, if not used in ex- 
cess. It has often been stated that acid dyes are faster to light than 
basic ones, but this is not uniformly true for leather. 

Chrome leather may be dyed in a manner similar to that of veg- 
etable tanned leather if it is first given a light surface retanning with 
a vegetable tanning material, such as gambier or sumac. The leather, 
after tanning and neutralizing, is drummed for a short time in a 
dilute tan liquor, washed, and then mordanted with a titanium or 
antimony salt, washed again and finally colored, after which it is 
fatliquored. Where the fatliquoring may cause a bleeding of the 
color, the dyeing must follow the fatliquoring. 

By the use of acid dyestuffs, chrome leather may be dyed directly, 
without the necessity for a vegetable retanning. In this case, how- 
ever, either the fatliquoring must be done before the coloring or else the 
color must be fixed in some way before fatliquoring. One method of 
accomplishing this is to divide the coloring operation into two parts, 
the leather being dyed first with an acid dye and then with a basic 
dye. Since the two kinds of dyestuffs coprecipitate each other from 
solution, forming insoluble lakes, they show little tendency to bleed 
into the fatliquor. . 

Direct colors may be used on either chrome or vegetable leathers. 
Alizarine and developed dyes find a use in the coloring of chrome 
leather. Sulfide dyes are frequently used where it is necessary to get a 
color very resistant to washing. For lists of the individual dyes and 
details of their application to leather, the reader should consult the 
various practical handbooks on leather dyeing. 

The shade of colored leathers may be darkened, or “saddened,” 
by drumming them in solutions of salts of iron, copper, or other heavy 
metals. In the production of black leathers, it is common to drum 
the leather first in a slightly alkaline solution of logwood extract and 
then in a solution of ferrous sulfate to develop the black. This is 
often topped by a second dyeing with some artificial black dyestuff. 


330 THE CHEMISTRY OF LEATHER MANUFACTURE 


After the leather has been dyed, it is customary to rinse it in 
cold water, smooth it out by slicking, lightly oil the surface, and then 
dry it. 

The chemistry of leather dyeing has not yet overtaken the art, 
although the process is primarily a chemical one. The effect obtained 
from a given color bath is a function of pH value, concentration, 
temperature, etc., but the literature contains no record of investiga- 
tions of leather dyeing under rigidly controlled conditions of pH 
value, etc., like those on tanning described in Chapters 11 to 14. One 
nae infer that similar principles are involved in both tanning and 

yeing. 


Finishing. 


For descriptions of the numerous mechanical operations and 
machines used in making leather, including splitting, shaving, slicking, 
samming, drying, staking, rolling, brushing, boarding, plating, glazing, 
and embossing, reference should be made to books treating the subject 
from a more obviously practical viewpoint, like those of Procter,’ 
Lamb,® and Rogers.® 

When the leather has been dried, after coloring, it is subjected 
to various mechanical operations in order to give it the desired physical 
properties. Ordinary shoe upper leather is coated with a size or 
finish in order to make it water repellent and more pleasing to the 
eye. These finishes usually have as a base an aqueous dispersion of 
gelatin, casein, blood albumin, egg albumin, gum tragacanth, or Irish 
moss, and are often mixed with colors or pigments. Another popular 
finishing material consists of a mineral pigment ground in a solution 
of shellac and borax in water; this is sprayed onto the grain surface 
of the leather by means of an atomizer. | 

Patent leathers are coated with a varnish made from boiled linseed 
oil, driers, and pigments. The leather is dried in a hot oven after 
the varnish has been applied. The surface is then rubbed smooth and 
a second coat applied, after which it is again dried in the oven. This 
may be repeated several times to get the desired effect. The surface 
of the leather is finally exposed to the sun or to ultra-violet rays for 
several hours. 

Suede or ooze calf leathers are made from the skins of slinks, 
chrome tanned, colored, and finished on the flesh side. The so-called 
buck leathers are made from chrome tanned cow hide, split to give a 
sufficiently thin layer, including the grain. The grain is buffed on 
an emery wheel and then dusted with a dry pigment. The names of - 
many of the commoner leathers indicate the method of finishing rather 
than the animal furnishing the skin. 
ee of Leather Manufacture. H. R. Procter. D. Van Nostrand, New York 


® Leather Dressing. M. C. Lamb. Leather Trades Publishing Co., London (1909). 
® Practical Tanning. A. Rogers. Henry Carey Baird & Co., New York (1922) a 


; AUTHOR INDEX 


Abderhalden, E., 66, 70 

Abt, G., 135-7, 141 

Alexander, J., 72 

Alsop, W. K., 228 

Andreasch, F., 147 

Andrews, J. C., 78 

Apostolo, C., 316 

Atkin, W. R., 68, 161, 166, 305-6 


Balderston, L., 279 

Baldracco, G., 135 

Baldwin, M. E., 80, 91, 92-3, 280-3, 
285-8 

Bassett, 305-6 


Bast, T. H., 8, 18 
Beans, H. T., 128 
Beck, A. J., 324 


Becker, H., 135-6, 179 
Benedict, A. J., 113 
Bergmann, M., 214 

Bien, Z., 108 

Blockey, F. A., 218 

Blockey, J. R., 306 

Bogue, R. H., 119-20, 126, 132 
Bonnet, F., 307 


Booge, J. E., 78 
Bowker, R. C., 324 
Brown, 200 


Burton, D., 306 
Burton, E. F., 231 
Buxton, B. H., 236 


Carmichael, T. B., 270 
Churchill, J. B., 324 
Conn. EK. J., 108 
Corran, J. W., 296 
Coulter, C. B., 108 
Cross, C. F., 270 
Cruess, W., 180 


Dakin, H. D., 70 

Daub, G., 8, 112, 116, 154, 169, 184-91, 
196-7 

Davidsohn, H., 108, 189 

Davis;-G.. E., 112-3 

Dean, A. L., 213 

Dekker, J., 205 

Dennis, Martin, 278 

Donnan, F. G., 

127-30 
Dumanski, A., 120 


94-9, I14, I2I-3, 


Eastlack, H. E., 128 
Eastman, G. W., 78-9 
Eitner, W., 135, 141, 201 
Elliott, F. A., 116-7, 143-4 
Emmett, A. D., 73 
Enna, Fini, 262, 319 
Evans, U. R., 270-1 

Ewald, A., 73 


Fahrion, W., 276-7, 317, 327 
Fales, H. A., 89 

Falk, K. G., 185, 189 
Fischer, Emil, 66, 213-4 
Fodor, A., 66, 108 

Foster, S. B., 231-9, 262, 295-6 
Freudenberg, K., 213-5 
Freundlich, H., 131, 236 
Frieden, A., 216-7 


Gage, S. H., 16 
Gallun, Albert F., Jr., 35, 111, 113, 
161, 166-72, 175-7, 188, 263, 268-9, 


324 

Gallun, Arthur H., 8, 14 
Gallun, Edwin A., 292-4 
Garelli, F., 316 

Gibbs, J. Willard, 131 
Gies, W. J., 73 

Graeb, 215 

Graham, Thomas, I19, 315 
Grasser, G., 231, 319-20 
Greenwood, C. V., 270 
Grineff, W., 108-9 
Gross, J., 108 


Hammarsten, O., 69 
Hampshire, P., 154 
Hardy, 3W.~ 8,,;. 109 
Harned, H. S., 80, 146 
Platt woken) 324 
Harvey, A., 205, 280 
Hassan, K., 230 

Hey, A. M., 319 
Higley, Parker, 114 
Hill, J. B. 321 
Hofmeister, F., 72-3, 105 
Hollander, C. S., 166 
Hoppenstedt, A. W., 256 
How Tek, 3i4 
Hough woe alesis 
Hull, M., 189 


332 


Jackson, D, D., 314 
Jettmar, J.°413 
Johnson, O. C., 108 
Jones, H. C., 78-9 


Kadish, H. L., 164-5 

Kadish, eVsitl, 4 1O4=n 

Kato, Y., 78 

Kelly, Margaret W., 8, 109, 113, 258-67, 
274-6, 285-92 : 

Kendall, J., 77-8 

Rem sh, J. 
265-6, 283-4 

Klaber, W., 283 

Knapp, F., 278 

Kohlrausch, F. W. G., 78-9 

Krall, L., 184-5 

Kraus, Cl AY 3 

Kihne, W., 73 


Lamb, M. C., 270, 280, 330 

Law, D. J., 160, 162-4, 181, 284 

Le Count, 70 

Lees A, B36 

Lewis, W. C. McC., 94, 296 

Lloyd, Dorothy J., 109, III, 114 

Loeb, Jacques, 8, 94, 104-6, 108-9, 112, 
114-6, 119-27, 130-2, I61 

Loeb, M. H.,:8 

[oie =} ahs ei ko 

Lumiére, A., 311 

Lumiere, L., 311 

Lundén, H., 77 


McBain, J. W., 1109, 130 
McCandlish, D., 8, 243 
McLaughlin, G. D.,, 146, 269 
Mandel, J. A., 69 
Marriott, R. H., 166 
Mendelssohn, A., 108 
Merryman, G. W., 321 
Meunier, L., 276-7, 316-9 
Michaelis, L., 108, 189, 206 
Moeller, W., 136, 324 
Morris, 200 
Mouries, M., 200 
Nance, C. W., 270 

_ Nelson, J. M., 89 

Y Neun obi 189 

\ Nihoul, E., 166 
\Northrop, J. H., 74, 187 
\Noyes, A: A., 78-9 


Oakes, Eee Vase 
Orchard, 149 
Ostwald, W., 77-8 


Pae§sler, J., 134-6 
Paniker, R., 214 
Parker, J. G., 271 


III-4, 208-11, 218-31, 257, 


AUTHOR ANDEX. 


Payne, M., 318 

Pechstein, H., 108 

Plimmer, R. H. A., 65, 73 

Poma, G., 89, 90 

Ponder, -C3-r4a 

Popp, G., 179 

Porter, Hs (tog 

Porter, R. E., 269 

Pottevin, H., 213 

Powarnin, G., 277 

Powis,. F., 127,° 330 

Procter, H. R, 8, 94, 98-104, 118-9, 
138, 140, 150, 161, 205, 217-8, 220, 
272-6, 279, 284, 297, 313, 330 

Pullman, E., 318 

Pullman, J., 318 


Ricevuto, A., 306 

Richards, T. W., 307 

Rideal, E. K., 149, 270-1 
Rogers, A., 330 

Rohm, O., 166, 174, 181, 314 
Romana, "Cp igs : 
Rona, P., 108, 206 
Rosenthal, G. J., 67-8, 181 
Ross, \H. Cree 


Salmon, C. S., 130 

Sand, Ho J Ss 9166 

Schattenfroh, A., 140-1 

Schiffss Hoare 

Schlichte, A. A., 163-4 

Schmidt, C. E., 134 

Schnurer, J., 141 

Schucht, H., 236 

Schultz, A., 278 

Schultz, G. W., 226 

Seshachalam, K:, 230 

seveik, Eouiat 

seyewetz, A. 311, 316-7 

Seymour-Jones, Alfred, 16, 35, 38, 139, 
141, 163, 166, 168, 181, 184, 262 

Seymour-Jones, Arnold, 140 

Seymour-Jones, Frank L., 8, 70, 73-4, 
143, 188, 197, 305-8 

Sheppard, S. E., 8, 113, 116-8, 142-4 

Sherman, H. C., 185, 189 

Siewert, 308 

Smith, \C, RiGya stro 

Smith, G. McP., 92 

Sommerhoff, E. O., 316 

Sorensen, S/) Pein te 

sosman, R. B., 78 

Stiasny, E., 163, 165, 214, 271, 319 

Strecker, A., 213-4 

Sweet, 9: 53 119 erie 


Takahashi, D., 108 

Teague, O., 236 

Thomas, Arthur W., 8, 14, 73, 74, 76-93, 
109, I13, 130, 188, I9I, 197, 216, 


AU LORAIN DEX, 


231-9, 256-67, 274-6, 280-3, 285-92, 
295-6, 324. 
Hac, 68, 


Thompson, 
305-6 

suuat, WU; J., 166, 315 

Tilley, F.:W., 141 

Trunkel, H., 274 

Turnbull, A., 270 


Van Tassel, E. D., Jr., 324 
Van Tieghem, P., 213 
Voitinovici, A., 70 
Vollbrecht, E., 215 


II9Q-20, 230, 


333 


Walden, P., 78, 213 

Walker, N., 70 

Wells, 70 

Whitmore, L. M., 324 

Whitney, 308 

Wilcox, W. H., 201 

Williams, O. J., 271 

Wilson, F. H., 180 

Wilson, Wynnaretta H., 99-106 

Wood, J. T., 147-8, 156, 160, 162-4, 
166, 174, 176, 179-81, 197, 199-201, 308 


Yocum, J: H. 135 


SUBJECT INDEX 


Acetic acid, 


effect upon vegetable tan liquors, | 


235 

ionization of, 77, 82 
Acids formed in drenching, 200 
Acids, order of strength, 79 
Acid unhairing, 166 
Adipose tissue, 23, 33 

vertical section of, 32 
Adsorption, 131-2 
Aging of leather, 228-30, 274-6 
Alanine, 65, 70 
Albino rat leather, grain surface of, 


of 
Albumin fixative, 18 
Albumins, 67-8 
isoelectric points of, 108 
Albuminoids, 67 
Albumose, action of 
189 
Aldehyde tanning, 318-9 
Aigarobilla, 206, 256 
Alligator leather, 243 
vertical section of, 251 
Aluminum salts, 
precipitation of vegetable tan liquor 
by, 236 
tanning with, 311-3 
Amino acids, 65-6, 70 
Ammonium chloride, effect upon 
bating, I90-I 
chrome tanning, 292-4 
pH value of acid solutions, 89-90 
pH value of chrome liquors, 283 
precipitation of chrome liquor by 
alkali, 284 
Ammonium hydroxide, 
ionization of, 78, 88 
unhairing with, 165 
Ammonium sulfate, effect upon 
pH value of acid solutions, 89-90 
pH value of chrome liquors, 282 
precipitation of chrome liquor by 
alkali, 284 
Amorphous tannins, 215 
Amylolytic enzymes, 174, 181, 200 
Inthrax, disinfection against, 139-41 
Attimony salts in mordanting leather, 


trypsin upon, 


: 329 
Arginine, 66, 70 
Arstnic disulfide, unhairing with, 157 


\ 


Arteries, 33, 38 

vertical section of, 32 
Aspartic acid, 65, 70 
Aspergillus niger, 213-5 
Astringency, 224, 231, 241, 260, 262 
Autoclave, leaching in, 208 


Babool bark, 206 

Bacillus anthracis, 139 
butyricus, 148 
coli, 174 
erodiens, 179 
fluorescens liquefaciens, 147, 149 
furfuris, 200 
gasoformans, 147 
liquidus, 147 
megeterium, 147, 201 
mesentericus fuscus, 147 
mesentericus vulgatus, 147 
mycoides, 147 
prodigiosus, 163 
subtilis, 147 

Bacterial action in 
bating, 179-80 
drenching, 200-1 
liming, 162-3 
pancreatin unhairing liquors, 167 
production of salt stains, 134-7 
soaking, 146-50 
sweating, I51-6 

Baldness, 31 

Barium chloride, effect upon 
pH value of acid solutions, 89-90 
pH value of chrome liquors, 283 
precipitation of vegetable tan liquor, 

235 
Beers hydroxide, ionization of, 78, 


Bases, order of strength, 79 
Bating, 173-98 
as a. hydrogen-ion concentration 
control, 178 
bacterial action in, 179-80 
effect of ammonium chloride, 190-1 
pancreatin, 189-90 
pH value, 185-9 
time, 188-90 
to delime skins, 179 
to digest collagen, 197-8 
to reduce plumping, 175 
to remove elastin, 181-97 


\ 334 


\ 


\ 
\ 


YUSIEC LINDE X 


Bating, with bacteria, 179-80 
with dung, 173, 184 
with pancreatin, 181-97 
Beam, 155 
Beamhouse, 142 
Beamster, 155 
Bear leather, grain surface of, 37 
Birch bark, 206 
Bismarck brown stain, 19 
Bismuth tanning, 316 
Blackheads, 31 
Bleaching leather, 322 
Blood, 75 
decolorizing tanning extracts with, 
211-2 
Bloom, 205, 215 
Boiling test in chrome tanning, 293-4 
Boric acid, ionization of, 77, 84 
Bran in drenching, 199 
Bromine tanning, 316-7 
Buck leather, 330 
Bulb, hair, 28, 30-64 
Butyric acid, ionization of, 77, 84 


Calcium chloride, precipitation of vege- 
table tan liquor by, 236 
Calcium hydroxide, 
ionization of, 79 
precipitation of vegetable tan liquors, 
210-1 
unhairing with, 156-60 
Calcium sulfarsenite, unhairing with, 


157 
Calf leather, 
grain surface of, 36, 197 
horizontal section through grain sur- 
face, 326 
vertical sections of, 2098, 299, 303 
(see also under leather) 
Calf skin, 48-53 
vertical sections of (before tanning), 
fresh, 49, 52, 53 
from lime liquor, 158, 159 
unhaired in pancreatin solutions, 
170, I71 
before bating, 182 
after bating, 183 
Camel leather, 243 
grain surface of, 37 
vertical section of, 253 
Canaigre, 206 
Carbol-xylene, 17 
Carbonic acid, ionization of, 77 
Casein, 
hydrolysis by trypsin, 189 
isoelectric point of, 108 
Caseinic acid, 66 
Catechins, 215 
Catechol tannins, 204 
Cells, 
epithelial, 23 


335 


Cells, 
fat; 33)'35, 51 
growth of, 25 
migratory, 35 
Cerealin, 200 
Cerium tanning, 316 
Chamois leather, 317 
Chemical constituents of skin, 65-75 
Chestnut wood, 206, 219-39, 256 
Chinese nutgalls, 214 
Chlorine tanning, 316-7 
Cholesterol, 75 
Chrome leather (see leather) 
Chrome liquors, 
dialysis of, 307 
diffusion into protein jellies, 284 
effect of added acid or alkali, 281 
dilution, 280 , 
neutral salts, 281-3 
salts of hydroxyacids, 297 
electrophoresis of, 305-8 
hydrolysis of, 280-4 
manufacture of, 278-9 
pH value of, 93; 280-97 
precipitation of, 283-4 
tanning with, 278-308 
ultrafiltration of, 307 
Chrome retan leather, 309 
vertical section of, 310 
Chrome tanning, 278-308 
boiling test in, 203-4 
effect of concentration, 288-92 
neutral salts, 292-6 
pH value, 285-97 
salts of hydroxyacids, 297 
time, 285-8 
theory of, 305-8 
Chromium collagenate, 279-80 
Chromium salts (see chrome liquors) 
Chromogenic bacteria, 135-6 
Citric acid, ionization of, 78, 85 
Clarifying tanning extracts, 211-2 
Clearing agent, I7 
Cod fish skin, 64 
vertical section of, 62 
Collagen, 35, 72-3 
combining weight, 273, 279-80 
compounds with chromium, 279-80, 
305 
tannin, 273-4 
fibers, 33, 35, 39-64 
Poa by acids or alkalis, 168, 
169 
enzymes, 73-4, 168, 197-8 
hot water, 72 
isoelectric point of, 100, 175-7 
relation to gelatin, 72-3 
two forms of, 109, 176 
Colloidal dispersions, 
tanning with, 316 
theory of stability of, 127-30 


336 


Coloring leather, 328-30 
Color value of tan liquor, effect of 
oxidation, 209-10 
pH value, 208-9 
Combining weight of collagen, 273, 
279-80 
gelatin, 102-3, 311 
Conductivity of gelatin dispersions, 120 
Connective tissue, 23, 33, 35, 38-64 
Co-operation between university and 
industry, 12-4 
Cordovan leather, 58, 243, 304 
grain surface of, 36 
vertical sections of, 246, 301 
Corneous layer of epidermis, 26-7, 39- 
64, 70, I51-4, 157-9, 169 
Cow hide, 39-48 
horizontal sections of, 43, 44, 45, 46, 


47 

vertical sections of, 40, 41, 192 
Cow leather, 

grain surface of, 36 

vertical section of, 310 
Curliness of hair, cause of, 29, 51 
Cutch wood, 206 
Cystine, 65, 70 


Dandruff, 25 
Decolorizing tanning extracts, 211-2 
Dehydrating specimens of skin, 17 
Deliming, 179, 199-203 
Depsides, 215 
Derma, 23, 33-64 
Dialysis of 

chrome liquors, 307 

vegetable tan liquors, 233 
Diastase of translocation, 200 
Diffusion into protein jellies of 

chrome liquor, 284 

vegetable tan liquor, 256-8 
Digallic acid, 213 
Disinfection of skin, 139-41 
Divi-divi, 206, 256 
Double refraction in gelatin disper- 

sions, I20 

Drenching, 199-202 
Dried forms of gelatin jellies, 144 
Drying of 

gelatin jellies, 143 

skins, 137-8 

tanning extracts, 212 
Dung, 

composition of, 174 

use of in bating, 173 
Dyestuffs, use of in coloring leather, 

328-30 


Ectoderm, 25. 
Edestin, isoelectric point of, 108 


Egg albumin, isoelectric point of, 108 


Elasticity of gelatin jellies, 118 


SUBJECT INDEX 


Elastin, 70-1 

hydrolysis by pancreatin, 181-97 
Elastin fibers, 35, 42, 182-3, 192-5 

removal in bating, 181-97 

effect upon leather, 196-7 

Electrical tanning, 270-1 
Electrophoresis of 

chrome liquors, 305-8 

vegetable tan liquors, 231-3 
Eleidin, 27, 29, 70 
Ellagic acid, 205, 215 
Emulsifying agents, 324 
Emulsin, 215 
Enzymes, 

action in bating, 181-91 

unhairing, 166-72 

activation of, IQI 

amylolytic, 174, 181, 200 

cerealin, 200 

emulsin, 215 

inactivation of, 187-91 

lipase, 174, I81 

list of in dung, 181 

pancreatin, 73-5, 166-72, 181-91 

pepsin, 73-5, 181 

rennin, 174, I81 

tannase, 215 

thrombase, 68, 172 

trypsin (see pancreatin) | 
Epidermal system, 25, 27-33, 39-64, 60 
Epidermis, 23, 25, 30-64 

horizontal section of, 43 

vertical section of, 26 
Epithelial cells, 23 

tissue, 23 
Erector pili muscle, 31, 39-60 
Erlicki fixer, 17 
Erodin, 179 


Falling of skin, 160, 175 
Fascia, superficial, 23, 33 
Fat cells, 33, 35, 39-58 
Fatliquoring, 323-5 
Fatty acid spews, 327-8 
Ferric chloride test for nontannin, 259 
Fertilizer from sulfide  unhairing 
liquors, 165 

Fiber sarcolemma, 35, 168 
Fibrin, 

hydrolysis by trypsin, 189 
Fibrinogen, 68 
Finishing leather, 330 
Fir bark, 206 
Fish skins, 62-4 
Fixative, albumin, 18 
Fixing specimens, 16-7 
Flesh, 23, 33 
Fleshing, 33, 142-50 
Follicle, hair, 29, 39-64 
Formic acid, 

absorption by skin, 140 


SUBJECT INDEX 


Formic acid, 

ionization of, 78, 82 

precipitation of vegetable tan liquor 

by, 235, 237-9 
use of in coloring leather, 329 
disinfecting skins, 139-41 

Freckles, 27 
Fuchsin stain, 19 


Gallic acid, 
effect upon tannin analysis, 
ionization of, 78, 83 
Gallotannic acid, 213-4 
Gambier, 206, 215-7, 219-39, 256-66, 276 
Gases formed in drenching, 200 
Gelatin, © 
combining weight of, 102-3, 31! 
hydrolysis of, 74-5 
isoelectric point of, 108 
mutarotation of, I12 
reaction with aluminum salts, 311 
chromium salts, 308-11 
tannin, 308 
relation to collagen, 72-3 
two forms of, 109-14 
Gelatin dispersions, 
absorption spectra of, 114 
conductivity of, 120 
double refraction of, 120 
osmotic pressure of, 120-5 
plasticity of, 126-7 
structure of, 118-20 
vapor pressure of, 120 
viscosity of, 112-3, 120-3, 125-7 
Gelatin jellies, 
diffusion of chrome liquor into, 284 
diffusion of vegetable tan liquor into, 
256-8 
dried forms of, 144 
drying of, 143 
elasticity of, 118 
reticulation of, 116-8 
structure of, 118-20 
swelling of, 98-123 
Gelatin-salt test for tannin, 216-7 
Glands, 27, 31, 33; 39-60 
Glassy layer of horse hide, 58 
Gliadin, isoelectric point of, 10 
Globulins, 67-8 
isoelectric points of, 108 
Glucose, use in reduction of sodium 
dichromate, 279 
Glucosides, 215 
Glue stock, 142 
Glutamic acid, 66, 70 
Glycine, 65, 70 
Goat leather, 304 
vertical section of, 302 
Goat skin, 56 
Gold sol, theory of stability of, 127-30 
Goose flesh, 31 


218-24 


337 
Grain membrane, 38 
Grain surface, 38 
horizontal section of from calf 
leather, 326 
loose, 149 


photomicrographs of from leathers, 
albino rat, 37 
bear, 37 
calf, 36, 197 
camel, 37 
cow, 36 
guinea-pig, 37 
hog, 30 
horned toad, 37 
horse, 36 
kid, 36 
salmon, 37 
_sheep, 36 
pipy, 149 
pitted, 39, 149, 154 
proteins of, 71 
rough, 146 
structure of, 326 
Guinea-pig leather, 60 
grain surface of, 37 
Guinea-pig skin, 60 
vertical section of, 61 


Hair, 27-31, 39-64 
bulb, 28, 39-64 
vertical section of, 28 
curliness, cause of, 29, 5! 
follicle, 29, 39-64 
papilla, 29, 48 
pigment, 31 
root, 29 
scales, 29 
photomicrograph of, 30 
shaft, 29 
Halibut skin, 64 
vertical section of, 62 
Handlers, in vegetable tanning, 241 
Hematoxylin-eosin stain, 18 
Hemi-celluloses, use of in tanning, 


270 
Hemlock bark, 206, 216, 219-39, 256, 
259-66, 274-5 | 
Hemoglobin, isoelectric point of, 108 
Hide powder, 
digestion by trypsin, 73-4 
preparation of, 72 
Hippopotamus leather, 243 
vertical section of, 255 
Histidine, 66, 70 
Histology of skin, 15-64 
Hog leather, 243 
grain surface of, 36 
vertical section of, 248 
Hog skin, 57 
vertical sections of, 57, 195 
Hooke’s law, 101, I! 


338 


Horned toad leather, 243 
grain surface of, 37 
vertical section of, 252 
Horse hide, 58-60 
vertical section of, 59 
Horse leather, 242-3, 304 
grain surface of, 36 
vertical sections of, 246, 247, 301 
Human skin, 21-39 
vertical sections from 
back, 22 
heel, 24, 26 
scalp, 21 
Hydration of ions, 91-3, 267, 292-6 
Hydrochloric acid, 


effect of neutral salts on pH value, 


ionization of, 78, 80 
precipitation of vegetable tan liquor 
by, 234, 237-9 _ 
Hydrogen-ion concentration, 
effect of neutral salts upon, 89-93, 
146, 281-4 
effect upon 
aldehyde tanning, 319 
bating, 185-9 
chrome tanning, 285-97 
color of vegetable tan liquors, 
208-9 
determination of tannin, 230-1 
diffusion into protein jellies of 
chrome liquor, 284 
vegetable tan liquor, 257-8 
fatliquoring, 325 
gelatin-salt test for tannin, 216-7 
hydrolysis of collagen by pancre- 
atin, 73-4, 168, 197-8 
elastin by pancreatin, 185-9 
leaching of vegetable tanning ma- 
terials, 208-9 
osmotic pressure of protein sys- 
tems, I2I-4 
oxidation of vegetable tan liquors, 
209-10 
plumping of skin, 175-7 
precipitation of vegetable tan 
liquors, 210-1, 237-9 
putrefaction, 150, 177 
stability of collagen-tannin com- 
pound, 265-6 
swelling of protein jellies, 98-114, 
175-7 
vegetable tanning, 262-7 
viscosity of gelatin dispersions, 
112-3, 120-3, 125-7 
values for different solutions (see 
pH values) 
Hydrolysis of 
chromium salts, 280-3 
collagen, 70-4, 168, 189, 197-8 
elastin, 70, 181-97 


SUBJECT INDEX 


Hydrolysis of 
keratin, 69-70, 151-4, 157-60, 164-72 
leather, 265-6, 274-6 ~ 
Hydroxyacids, salts of, effect upon 
chrome tanning, 297 


Imbedding specimens of skin, 17 
Inactivation of enzymes, 187-91 
Interfibrillary cementing substance, 68 
Spee of acids and bases (tables), 
O- 
effect of neutral salts, 89-93 
temperature, 79 

order of strengths, 79 

tables showing pH values, 80-8 
Iron tanning, 313-4 
Isoelectric points of 

collagen, 109, 175-7 

gelatin, 108, 109-14 

other proteins, 108 

tannins, 233 
Isoleucine, 65 


Keratin, 29, 60 
hydrolysis of, 69-70, I5I-4, 157-60, 
164-72 
Keratohyalin, 70 
Keto-enol tautomerism in 
proteins, 110, 114 
tannins, 277 
Kid leather, 56 
grain surface of, 36 
Kid skin, 56 
vertical section of, 55 
Kips, 133 


Lactic acid, 
effect upon plumping of skin in 
vegetable tan liquor, 268-9 
ionization of, 78, 83 
precipitation of vegetable tan liquor 
by, 235 
Lamb fur, 313 
vertical section of, 312 
Larch bark, 206, 216, 219-39, 256, 259, 
261-4 
Layers, in vegetable tanning, 241-2 
Leaching of vegetable tanning mate- 
rials, 207-9 
Leather, 
action of sulfuric acid on, 323 
aging of, 228-30, 274-6 
alum, 311-3 
ancient, II 
bleaching, 322 
chamois, 317 
chrome, 278-308 
comparison with vegetable leather, 
298-304 
chrome retan, 309-10 
coloring, 328-30 


SUBJECT INDEX 


Leather, 
decomposition by hot alcohol, 274-6 
fatliquoring, 323-5 
finishing, 330 
grain surface of (see grain surface) 
horizontal section of, 326 
hydrolysis of, 265-6, 274-6 
loading, 242 
mordanting, 329 
oxidation of fat in, 328 
resistance to washing, 225 
spew on, 327-8 
tensile strength of, 324 
vegetable, 240-77 
comparison with chrome leather, 
298-304 
vertical sections of, 
alligator, vegetable, 251 
calf, chrome, 299 
vegetable, 208 
calf slink, chrome, 303 
camel, vegetable, 253 
cow, chrome retan, 310 
goat, chrome, 302 
hippopotamus, vegetable, 25 
hog, vegetable, 248 ; 
horned toad, vegetable, 252 
horse, chrome, 301 
vegetable, 246, 247 
lamb slink, alum, 312 
salmon, vegetable, 249 
shark, vegetable, 250 
sheep, vegetable, 245 
steer, vegetable, 244 
walrus, vegetable, 254 
Lecithins, 75 
Leucine, 65, 70 
Ligamentum nuchae, 70-1 
Lignosulfonic acids, 320 
Limed skin, washing of, 155 
Lime liquors, 
bacterial action in, 162-3, 166 
pH value of, 93, 157, 162, 199 
unhairing with, 156-60 
Lipase, 174 
Lithium chloride, effect upon 
chrome tanning, 292-4 
pH value of acid solutions, 89-90 
chrome liquors, 283 
Logwood, 
use in coloring leather, 329 
stains for skin sections, 18-9 
Lymph, 75 
ducts, 35 
Lysine, 66, 70 


Magnesium chloride, effect upon 
chrome tanning, 292-4 
pH value of acid solutions, 89-90 
precipitation of chrome liquor by 
alkali, 284 


339 


Magnesium sulfate, effect upon 
pH value of acid solutions, 89-90 
chrome liquor, 282 
precipitation of chrome liquor by 
alkali, 284 
Mallet bark, 206 
Malpighian layer of epidermis, 25-6, 
70, I51-72 
destruction by acids, 166 
alkalis, 156-65 
bacteria, 151-6, 162-3, 166 
enzymes, 166-72 


_ Mangrove bark, 206, 256 


Melanins, 27, 67, 69 
Mellow lime liquors, 162 
Membrane equilibria, 94-132 
potentials, 96-8, 123-5 
Mercuric chloride, 
absorption by skin, 140 
use of in disinfecting skins, 139-41 
Mesoderm, 25 
Micrococcus flavus liquefaciens, 163 
Microtomy, 18 
Migratory cells, 35 
photomicrograph of, 34 
Mimosa bark (see wattle bark) 
Moellon degras, 317 
Mordanting leather, 320 
Mounting sections, 19-20 
Mucins, 67-9 
Mucoids, 69 
Muscles, 23, 31, 39-60 
Muscular tissue, 23 
Mutarotation of gelatin, 112 
Myrobalans, 206, 256 


Nemathelminthes, 154 
Nerves, 31, 33, 38 

vertical section of, 32 
Nervous tissue, 23 
Neutral salt effect, 89-93, 146, 281-4 
Nitric acid, ionization of, 78, 80 
Nucleic acid, isoelectric point of, 108 
Nucleus of a cell, 25, 46 
Nutgalls, 214 


Oak bark, 206, 215-6, 219-39, 256, 259-69 
Oak wood, 206 
Oil tanning, 317-8 
Ooze leather, 304, 330 
vertical section of, 303 
Open-vat leaching, 207 
Oropon, I81 
Osage orange wood, 206, 219-39 
Osmotic pressure of protein systems, 
120-5 
effect of pH value, 121-4 
neutral salts, 122-4 
Oxalic acid, ionization of, 78, 86 
Oxidation of _ 
fat in leather, 328 
vegetable tan liquors, 209-10 


340 


-Oxybenzophenones, 215 
Oxyhemoglobin, isoelectric point of, 


10 
Oxyphenylstyrylketones, 215 
Oxyproline, 66, 70 


Palmetto, 206 
Pancreatin, 
action upon 
albumose, 189 
casein, 189 
collagen, 73-4, 168, 197-8 
elastin, 181-97 
fibrin, 189 
gelatin, 74-5 
hide powder, 73-4 
skin, 166-72, 181-98: 
bating with, 169-72, 181-97 
unhairing with, 166-72 
Pancreol, 184 
Papilla, 25, 38, 42, 58, 256 
hair, 29 
Pars papillaris, 26 
Patent leather, 330 
Penicilium glaucum, 328 
Pentacetyl glucose, 214 
Pentadigalloylglucose, 214 
Pepsin, 73-5, 181 
Peptones, 65 
Persian lamb fur, 313 
vertical section of, 312 
Phenylalanine, 65, 70 
Phlobaphenes, 205, 215 
Phloroglucin tannins, 215 
Phosphoric acid, 
as a buffer in bating, 184-6 
ionization of, 78, 81 
precipitation of vegetable tan liquor 
by, 235 
Photomicrography, 20 
Physical chemistry of the proteins, 94- 
132 
pH value, 
acids (tables), 80-6 
aldehyde tan liquors, 319 
bases (tables), 87-8 
bate liquors, 93, 176-9, 184-91, 197, 
199 
bleaching solutions, 322 
chrome liquors, 93, 280-97 
definition of, 80 
effect of (see hydrogen-ion concen- 
tration ) 
fatliquors, 93, 325 
lime liquors, 93, 157, 162, 199 
pancreatin unhairing liquors, 167 
pickle liquors, 93, 203 
syntan solutions, 320 
vegetable tan liquors, 93, 199, 208-11, 
237-9, 257-8, 260-76 
Pickling, 138-9, 202 


SUBJECT INDEX 


Picro-indigo-carmine stain, 19 
Picro-red stain, 19 
Pigment, 27, 31, 69 
Pilo-motor nerves, 31 
Pipy grain, 149 
Pitted grain, 39, 149, 154 
Plasticity of gelatin dispersions, 126-7 
Plate cultures of 
puer liquor, 180 
soak water, 148 
Plumping of skin, 160 
effect of lactic acid, 268-9 
pH value, 175-7 
sodium chloride, 268-9 
in vegetable tan liquor, 268-9 
measurement of degree of, 161 
reduction in bating, 175 
two points of minimum, 175-7 
Polypeptides, 65-7 
Pores, 27 
Potassium bromide, effect upon pre- 
cipitation of chrome liquor by 
alkali, 284 
Potassium chloride, effect upon 
pH value of acid solutions, 89-90 
chrome liquors, 283 
precipitation of chrome liquor by 
alkali, 284 
vegetable tan liquor, 234 
Foe hydroxide, ionization of, 79, 


Potassium nitrate, effect upon precipi- 
tation of chrome liquor by 
alkali, 284 | 

Potential difference of 

colloidal dispersions, 127-30 

protein systems, 114-6 

vegetable tan liquors, 231-3 
Precipitation of 

chrome liquors, 283-4 

fatliquors, 325 

sols, theory of, 127-30 

vegetable tan liquors, 209-11, 233-9 

Preservation of skin, 133-9 

Prickles, 25 

Proline, 66, 70 

Protein-acid equilibria, 98, et sequ. 

Protein-alkali equilibria, 107, et sequ. 

Protein jellies, 

diffusion of chrome liquor into, 284 
vegetable tan liquor into, 256-8 

drying of, 143 

elasticity of, 118 

rhythmic swelling of, 116-8, 201, 241 

structure of, 118-20 

swelling of, 98-123, 160, 175-7, 268-9 

Proteins, 

classification of, 67 

hydrolytic products of, 65-6, 70 

isoelectric points of (table), 108 

physical chemistry of, 94-132 


SUBJECT 


Proteins, 
synthesis of, 66 
Proteoses, 65 
Proteus mirabilis, 148 
vulgaris, 148 
Protoplasm, 25, 70 
Puering (see bating) 
Puer liquor, plate culture of, 180 
Putrefaction, effect of pH value, 150, 
177 
Putrid soak, 149 
Pyrogallol tannins, 204 


Quebracho wood, 206, 215-6, 219-39, 
256-65 
Quinol, 276-7, 319 
Quinones, 276-7 
tanning with, 318-9 


Rapid tanning, 270-1 

Rat leather, grain surface of, 37 

Reds, 205, 215 

Rennin, 174 

Resistance of leather to washing, 225 

Resorcin-fuchsin stain, 19 

Reticular layer of skin, 39-64 
vertical section of, 34 

Reticulation of photographic negative, 


Ti7 

Rhythmic swelling of protein jellies, 
116-8, 201, 241 

Rochelle salt, effect upon chrome tan- 
ning, 297 

Root, hair, 29 

Run pelts, 154 


Salicylic acid, ionization of, 78, 86 
Salmon leather, 243 

grain surface of, 37 

vertical section of, 249 
Salmon skin, 64 

vertical section of, 62, 63 
Salting out, theory of, 127-30 
Salting skins, 133-8 
Salt stains, 134-7 
Scales, hair, 29 
Scalp, vertical section of, 21 
Scudding, 155 
Sebaceous ducts, 44 

glands, 27, 31, 39-60 
Sebum, 75 
Sectioning, 18 
Serine, 65, 70 
Serum albumins and globulins, isoelec- 

tric points of, 108 

Shaft, hair, 29 
Shark leather, 243 

vertical section of, 250 
Sharpening agents in liming, 157 


INDEX 


Sheep leather, 242 
grain surface of, 36 
vertical section of, 245 
Sheep skin, 51-6 
vertical sections of, 
fresh, 54 
from sweat chamber, 152, 153 
before bating, 194 
Shell of horse hide, 58 
Sides, 145 
Silicic acid tanning, 315 
Skin, 
chemical constituents of, 65-75 
dehydrating, 17 
disinfection of, 139-41 
drying of, 137-8 
falling of, 160-2, 175-7 
fixing, 16-7 
histology of, 15-64 
imbedding, 17 
plumping of, 160-2, 175-7, 268-9 
preservation of, 133-9 
salting, 133-8 
sampling, 16 
sectioning, 18 
sections and descriptions of skins 
from 
alligator, 64 
calf, 48-53, 158-9, 170-1, 193 
camel, 64 
cod fish, 62, 64 
cow, 39-48, 192 
goat, 56 
guinea-pig, 60-1 
halibut, 62, 64 
hippopotamus, 64 
hog, 57, 195 
horned toad, 64 
horse, 58-60 
human, 21-39 
kid, 55, 56 
salmon, 62-4 
shark, 64 
sheep, 54, 152-3, 194 
steer, 39 
walrus, 64 
Soaking, 142-50 
Soak, putrid, 149 
Soak water, plate culture of, 148 
Sodium carbonate, as a preventive of 
salt stains, 134-7 
Sodium chloride, effect upon 
chrome tanning, 292-6 
pH value of acid solutions, 89-93 
chrome liquors, 282-3 
plumping of skin in vegetable tan 
liquor, 268-9 
precipitation of chrome liquor by 
alkali, 284 
vegetable tanning, 266-7 
Sodium dichromate, 278-9 


341 


342 


Sodium fluoride, as a 
salt stains, 135 
Sodium hydroxide, 
as an unhairing agent, 164 
effect upon hydrolysis of chrome 
liquor, 281 
ionization of, 79, 87 
Sodium sulfate, effect upon 
chrome tanning, 294-6 
pH value of acid solutions, 89-90 
chrome liquors, 282 
precipitation of chrome liquor by 
alkali, 284 
vegetable tanning, 266-7 
Sole leather, 242 
vertical section of, 244 
Spew on leather, 327-8 
Spruce bark, 206 
Spruce extract, 321 
Stability of 
chrome liquors, 283-4 
collagen-tannin compound at differ- 
ent pH values, 265-6 
colloidal dispersions, 127-30 
fatliquors, 325 
vegetable tan liquors, 209-11, 233-9 
Staining sections, 18-9 
Stearamid, 324 
Steer leather, 242 
vertical section of, 244 
Sticks, in vegetable tanning, 241 
Stratum corneum, 26-7, 39-64, 70, 151-4, 
157-9, 169 
granulosum, 26-7, 70 
lucidum, 26-7 
mucosum, 25-7, 70, I51-72 
Stripping chrome leather, 297 
vegetable leather, 265-6, 322 
Stuffing, 323-5 
Sucrose, effect upon chrome tanning, 
295-6 
Sudoriferous glands, 27, 31, 33, 39-60 
Suede leather, 303-4, 330 
Sulfide unhairing liquors, 
from, 165 
Sulfite cellulose, 271, 321 
Sulfonated oils, 323 
Sulfuric acid, 
action upon leather, 323 
combination with skin during chrome 
tanning, 285-92 
effect of neutral salts upon the pH 
value of solutions of, ‘90-1 
effect upon the hydrolysis of chrome 
liquor, 281 
ionization of, 78, 81 
precipitation of vegetable tan liquor 
by, 234, 237-9 é 
Sulfurous acid, use of in soaking, 150 
Sulfur tanning, 316 
Sumac, 206, 219-39, 256 


preventive of 


fertilizer 


SUBJECT INDEX 


Superficial fascia, 23, 33 
Sweat ducts, 27 
glands, 27, 31, 33, 39-60 
Sweating process of unhairing, 151-6 
Swelling of protein jellies, 98-123, 160, 
175-7; 268-9 
effect- of 
neutral salts, 106-7, 122, 200, 202, 
268-9 
pH value, 98-114, 175-7 
polyvalent ions, 106, 108, 161 
family of curves of, 105 
rhythmic swelling, 116-8, 201, 241 
two points of minimum, 109-14, 175-7 
Syntans, 271, 319-20 
Synthesis of 
proteins, 66 
tannins, 214 


Tan liquors (see vegetable tan liquors) 
Tannase, 215 
Tanning, evolution of art of, 11-2 
Tanning extracts, manufacture of, 207- 
12 
Tanning with 
aldehydes, 318-9 
aluminum salts, 311-3 
bismuth salts, 316 
bromine, 316-7 
cerium salts, 316 
chlorine, 316-7 
chromium salts, 278-308 
colloidal dispersions, 316 
iron salts, 313-4 
oils, 317-8 
quinones, 318-9 
silicic acid, 315 
sulfite cellulose, 321 
sulfur, 316 
syntans, 319-20 
vegetable tanning materials, 240-77 
Tannins, 204, 213-39 
amorphous, 215 
catechol, 204, 215 
definition of, 215-6 
determination of, 217-31 
electrical charge on, 231-3 
formation from nontannin, 226 
gelatin-salt test for, 216-7 
isoelectric points of, 233 
mordanting chrome leather with, 329 
oxidation of, 209-10, 215 
phloroglucin, 215 
precipitation of, 209-11, 233-9 
purification of, 214 | . 
pyrogallol, 204, 215 
qualitative tests for, 205, 216-7 
synthesis of, 214 
Tartaric acid, ionization of, 78, 85 
Tensile strength, effect of grease upon, 
324 


Su DIE Iielly EX 


Thermostat layer of skin, 31, 39-64 
horizontal sections of, 44, 45, 46, 47 
vertical sections of, 41, 153, 159, 170, 

171, 192, 193, 194, 195 

Thrombase, 68, 172 

Thumb print, 38 

Titanium salts in mordanting leather, 

320 

Trimming, 142 

Trypsin (see pancreatin) 

Tryptophane, 66 

Tuberin, isoelectric point of, 108 

Tyrosine, 65, 70 


Ultrafiltration of chrome liquors, 307 
Unhairing with 
acids, 166 
ammonium hydroxide, 165 
arsenic disulfide, 157 
bacteria, 151-6, 162-3, 166 
calcium hydroxide, 156-60 
calcium sulfarsenite, 157 
enzymes, 166-72 
sodium hydroxide, 164 
sodium sulfide, 157, 164 
sweating, 151-6 
University, co-operation with industry, 
12-4 


Vacuum tanning, 270 
Valine, 65, 70 
Valonia, 206, 256 
Vapor pressure of gelatin dispersions, 
120 
Vegetable leather (see leather) 
Vegetable tan liquors, 
color value of, 208-10 
dialysis of, 233 
diffusion into protein jellies, 256-8 
electrophoresis of, 231-3 
oxidation of, 209-10 


343 


Vegetable tan liquors, 
plumping of skin in, 268-9 
potential difference of, 231-3 
precipitation of, 209-II, 233-9 
Vegetable tanning, 240-77 
effect of concentration, 258-62 
neutral salts, 266-7 
nontannin, 257 
pH value, 262-5 
time, 258-62 
rate increased by 
electricity, 270-1 
hemi-celluloses, 270 
nontannin, 241, 271 
sulfite cellulose, 271 
syntans, 271 
vacuum, 270 
theory of, 271-7 
Vegetable tanning materials, 204-12 
classification, 204-5 
leaching of, 207 
effect of various electrolytes, 210- 
at 
sources, 205-7 
Veins, 33, 38 
vertical section of, 32 
Viscosity of gelatin dispersions, 
120-3, 125-7 


112-3, 


Walrus leather, 243, 256 
vertical section of, 254 
Washing of limed skin, effect of time 
of, 155 
Wattle bark, 206, 216, 225-39, 259-64 
Willow bark, 206 
Wool, mune eae of segment of, 


Wool aaliee 138 


Zinc chloride, as a preventive of salt 
stains, 134 


att 


‘gic 


